##### SUMMARY
Certain `requirements.yml` file syntax still leads to traceback.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
lib//ansible/cli/galaxy.py

##### ANSIBLE VERSION

```paste below
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 2.10.0.dev0
  config file = None
  configured module search path = ['/Users/alancoding/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /Users/alancoding/Documents/repos/ansible/lib/ansible
  executable location = /Users/alancoding/.virtualenvs/awx_collection/bin/ansible
  python version = 3.6.5 (default, Apr 25 2018, 14:23:58) [GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.1)]
```

##### CONFIGURATION
```paste below
ansible-config dump --only-changed
[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_NOCOWS(env: ANSIBLE_NOCOWS) = True
```

##### OS / ENVIRONMENT
Mac OS / N/A


##### STEPS TO REPRODUCE
Make a requirements file that has a null entry for collections

```yaml
collections:
```

now run `ansible-galaxy collection install -r requirements.yml -vvv`

##### EXPECTED RESULTS
Does nothing, exit code 0


##### ACTUAL RESULTS
traceback

```paste below
Traceback (most recent call last):
  File "/Users/alancoding/Documents/repos/ansible/bin/ansible-galaxy", line 123, in <module>
    exit_code = cli.run()
  File "/Users/alancoding/Documents/repos/ansible/lib/ansible/cli/galaxy.py", line 456, in run
    context.CLIARGS['func']()
  File "/Users/alancoding/Documents/repos/ansible/lib/ansible/cli/galaxy.py", line 928, in execute_install
    requirements = self._require_one_of_collections_requirements(collections, requirements_file)
  File "/Users/alancoding/Documents/repos/ansible/lib/ansible/cli/galaxy.py", line 662, in _require_one_of_collections_requirements
    requirements = self._parse_requirements_file(requirements_file, allow_old_format=False)['collections']
  File "/Users/alancoding/Documents/repos/ansible/lib/ansible/cli/galaxy.py", line 550, in _parse_requirements_file
    for collection_req in file_requirements.get('collections', []):
TypeError: 'NoneType' object is not iterable
```

documentation_url='http://docs.example.com',
                homepage_url='http://example.com',
                min_ansible_version=ansible_version[:3],  
                dependencies=[],
            ))
            obj_path = os.path.join(init_path, obj_name)
        elif galaxy_type == 'collection':
            namespace, collection_name = obj_name.split('.', 1)
            inject_data.update(dict(
                namespace=namespace,
                collection_name=collection_name,
                version='1.0.0',
                readme='README.md',
                authors=['your name <example@domain.com>'],
                license=['GPL-2.0-or-later'],
                repository='http://example.com/repository',
                documentation='http://docs.example.com',
                homepage='http://example.com',
                issues='http://example.com/issue/tracker',
                build_ignore=[],
            ))
            obj_path = os.path.join(init_path, namespace, collection_name)
        b_obj_path = to_bytes(obj_path, errors='surrogate_or_strict')
        if os.path.exists(b_obj_path):
            if os.path.isfile(obj_path):
                raise AnsibleError("- the path %s already exists, but is a file - aborting" % to_native(obj_path))
            elif not force:
                raise AnsibleError("- the directory %s already exists. "
                                   "You can use --force to re-initialize this directory,\n"
                                   "however it will reset any main.yml files that may have\n"