### Summary

If `dependencies` in `galaxy.yml` is defined but not assigned any value, the installation would fail.


Please note the `dependencies`.

The installation would fail.

```yml
namespace: my_namespace
name: my_collection
version: 1.0.0
readme: README.md
authors:
- your name <example@domain.com>
description: your collection description
license:
- GPL-2.0-or-later
license_file: ''
tags: []
dependencies:
repository: http://example.com/repository
documentation: http://docs.example.com
homepage: http://example.com
issues: http://example.com/issue/tracker
build_ignore: []
```

It works if `dependencies` is assigned to `{}`.

```yml
namespace: my_namespace
name: my_collection
version: 1.0.0
readme: README.md
authors:
- your name <example@domain.com>
description: your collection description
license:
- GPL-2.0-or-later
license_file: ''
tags: []
dependencies: {}
repository: http://example.com/repository
documentation: http://docs.example.com
homepage: http://example.com
issues: http://example.com/issue/tracker
build_ignore: []
```

### Issue Type

Bug Report

### Component Name

ansible-galaxy

### Ansible Version

```console
$ ansible --version
[WARNING]: You are running the development version of Ansible. You should only run Ansible from "devel" if you are modifying
the Ansible engine, or trying out features under development. This is a rapidly changing source of code and can become
unstable at any point.
ansible [core 2.14.0.dev0] (devel abdd237de7) last updated 2022/04/19 11:12:55 (GMT +800)
  config file = None
  configured module search path = ['/Users/jackzhang/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /Users/jackzhang/src/github/ansible/lib/ansible
  ansible collection location = /Users/jackzhang/.ansible/collections:/usr/share/ansible/collections
  executable location = /Users/jackzhang/.pyenv/versions/v-397/bin/ansible
  python version = 3.9.7 (default, Oct 28 2021, 19:01:53) [Clang 12.0.5 (clang-1205.0.22.11)]
  jinja version = 3.1.1
  libyaml = True
```


### Configuration

```console
$ ansible-config dump --only-changed
CONFIG_FILE() = None
```


### OS / Environment

MacOS

### Steps to Reproduce

<!--- Paste example playbooks or commands between quotes below -->
```shell
$ ansible-galaxy collection init my_namespace.my_collection
$ cd my_namespace.my_collection
$ gsed -ri 's/^(dependencies:)(.*)$/\1/' galaxy.yml
$ ansible-galaxy collection install -vvv .
```


### Expected Results

I expect the `my_namespace.my_collection` with `dependencies=None` can be installed successfully

### Actual Results

```console
ansible-galaxy collection install -vvvv .
[WARNING]: You are running the development version of Ansible. You should only run Ansible from "devel" if you are modifying
the Ansible engine, or trying out features under development. This is a rapidly changing source of code and can become
unstable at any point.
ansible-galaxy [core 2.14.0.dev0] (devel abdd237de7) last updated 2022/04/19 11:12:55 (GMT +800)
  config file = None
  configured module search path = ['/Users/jackzhang/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /Users/jackzhang/src/github/ansible/lib/ansible
  ansible collection location = /Users/jackzhang/.ansible/collections:/usr/share/ansible/collections
  executable location = /Users/jackzhang/.pyenv/versions/v-397/bin/ansible-galaxy
  python version = 3.9.7 (default, Oct 28 2021, 19:01:53) [Clang 12.0.5 (clang-1205.0.22.11)]
  jinja version = 3.1.1
  libyaml = True
No config file found; using defaults
Starting galaxy collection install process
Found installed collection geerlingguy.testing:1.0.0 at '/Users/jackzhang/.ansible/collections/ansible_collections/geerlingguy/testing'
Found installed collection lablabs.wireguard:0.1.1 at '/Users/jackzhang/.ansible/collections/ansible_collections/lablabs/wireguard'
[WARNING]: Collection at '/Users/jackzhang/.ansible/collections/ansible_collections/foo/bar' does not have a MANIFEST.json
file, nor has it galaxy.yml: cannot detect version.
Found installed collection foo.bar:* at '/Users/jackzhang/.ansible/collections/ansible_collections/foo/bar'
Found installed collection robertdebock.development_environment:2.1.1 at '/Users/jackzhang/.ansible/collections/ansible_collections/robertdebock/development_environment'
Found installed collection containers.podman:1.9.3 at '/Users/jackzhang/.ansible/collections/ansible_collections/containers/podman'
Found installed collection community.docker:2.3.0 at '/Users/jackzhang/.ansible/collections/ansible_collections/community/docker'
Found installed collection community.general:4.7.0 at '/Users/jackzhang/.ansible/collections/ansible_collections/community/general'
Found installed collection community.molecule:0.1.0 at '/Users/jackzhang/.ansible/collections/ansible_collections/community/molecule'
Process install dependency map
ERROR! Unexpected Exception, this is probably a bug: 'NoneType' object has no attribute 'items'
the full traceback was:

Traceback (most recent call last):
  File "/Users/jackzhang/src/github/ansible/lib/ansible/cli/__init__.py", line 601, in cli_executor
    exit_code = cli.run()
  File "/Users/jackzhang/src/github/ansible/lib/ansible/cli/galaxy.py", line 646, in run
    return context.CLIARGS['func']()
  File "/Users/jackzhang/src/github/ansible/lib/ansible/cli/galaxy.py", line 102, in method_wrapper
    return wrapped_method(*args, **kwargs)
  File "/Users/jackzhang/src/github/ansible/lib/ansible/cli/galaxy.py", line 1300, in execute_install
    self._execute_install_collection(
  File "/Users/jackzhang/src/github/ansible/lib/ansible/cli/galaxy.py", line 1328, in _execute_install_collection
    install_collections(
  File "/Users/jackzhang/src/github/ansible/lib/ansible/galaxy/collection/__init__.py", line 683, in install_collections
    dependency_map = _resolve_depenency_map(
  File "/Users/jackzhang/src/github/ansible/lib/ansible/galaxy/collection/__init__.py", line 1594, in _resolve_depenency_map
    return collection_dep_resolver.resolve(
  File "/Users/jackzhang/.pyenv/versions/3.9.7/envs/v-397/lib/python3.9/site-packages/resolvelib/resolvers.py", line 453, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "/Users/jackzhang/.pyenv/versions/3.9.7/envs/v-397/lib/python3.9/site-packages/resolvelib/resolvers.py", line 347, in resolve
    failure_causes = self._attempt_to_pin_criterion(name, criterion)
  File "/Users/jackzhang/.pyenv/versions/3.9.7/envs/v-397/lib/python3.9/site-packages/resolvelib/resolvers.py", line 207, in _attempt_to_pin_criterion
    criteria = self._get_criteria_to_update(candidate)
  File "/Users/jackzhang/.pyenv/versions/3.9.7/envs/v-397/lib/python3.9/site-packages/resolvelib/resolvers.py", line 198, in _get_criteria_to_update
    for r in self._p.get_dependencies(candidate):
  File "/Users/jackzhang/src/github/ansible/lib/ansible/galaxy/dependency_resolution/providers.py", line 425, in get_dependencies
    for dep_name, dep_req in req_map.items()
AttributeError: 'NoneType' object has no attribute 'items'
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
import copy
import json
import os
import pytest
import re
import shutil
import stat
import tarfile
import yaml
from io import BytesIO, StringIO
from mock import MagicMock, patch
from unittest import mock
import ansible.module_utils.six.moves.urllib.error as urllib_error
from ansible import context
from ansible.cli.galaxy import GalaxyCLI
from ansible.errors import AnsibleError
from ansible.galaxy import collection, api, dependency_resolution
from ansible.galaxy.dependency_resolution.dataclasses import Candidate, Requirement
from ansible.module_utils._text import to_bytes, to_native, to_text
from ansible.module_utils.common.process import get_bin_path
from ansible.utils import context_objects as co
from ansible.utils.display import Display
class RequirementCandidates():
    def __init__(self):