##### 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
```

elif not collections and not requirements_file:
                raise AnsibleError("You must specify a collection name or a requirements file.")
            if requirements_file:
                requirements_file = GalaxyCLI._resolve_path(requirements_file)
            requirements = self._require_one_of_collections_requirements(collections, requirements_file)
            output_path = GalaxyCLI._resolve_path(output_path)
            collections_path = C.COLLECTIONS_PATHS
            if len([p for p in collections_path if p.startswith(output_path)]) == 0:
                display.warning("The specified collections path '%s' is not part of the configured Ansible "
                                "collections paths '%s'. The installed collection won't be picked up in an Ansible "
                                "run." % (to_text(output_path), to_text(":".join(collections_path))))
            output_path = validate_collection_path(output_path)
            b_output_path = to_bytes(output_path, errors='surrogate_or_strict')
            if not os.path.exists(b_output_path):
                os.makedirs(b_output_path)
            install_collections(requirements, output_path, self.api_servers, (not ignore_certs), ignore_errors,
                                no_deps, force, force_deps, context.CLIARGS['allow_pre_release'])
            return 0
        role_file = context.CLIARGS['role_file']
        if not context.CLIARGS['args'] and role_file is None:
            
            raise AnsibleOptionsError("- you must specify a user/role name or a roles file")
        no_deps = context.CLIARGS['no_deps']
        force_deps = context.CLIARGS['force_with_deps']
        force = context.CLIARGS['force'] or force_deps
        roles_left = []
        if role_file:
            if not (role_file.endswith('.yaml') or role_file.endswith('.yml')):
                raise AnsibleError("Invalid role requirements file, it must end with a .yml or .yaml extension")
            roles_left = self._parse_requirements_file(role_file)['roles']