##### 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
```

galaxy_type = 'collection' if parser.metavar == 'COLLECTION_ACTION' else 'role'
        args_kwargs = {}
        if galaxy_type == 'collection':
            args_kwargs['help'] = 'The collection(s) name or path/url to a tar.gz collection artifact. This is ' \
                                  'mutually exclusive with --requirements-file.'
            ignore_errors_help = 'Ignore errors during installation and continue with the next specified ' \
                                 'collection. This will not ignore dependency conflict errors.'
        else:
            args_kwargs['help'] = 'Role name, URL or tar file'
            ignore_errors_help = 'Ignore errors and continue with the next specified role.'
        install_parser = parser.add_parser('install', parents=parents,
                                           help='Install {0}(s) from file(s), URL(s) or Ansible '
                                                'Galaxy'.format(galaxy_type))
        install_parser.set_defaults(func=self.execute_install)
        install_parser.add_argument('args', metavar='{0}_name'.format(galaxy_type), nargs='*', **args_kwargs)
        install_parser.add_argument('-i', '--ignore-errors', dest='ignore_errors', action='store_true', default=False,
                                    help=ignore_errors_help)
        install_exclusive = install_parser.add_mutually_exclusive_group()
        install_exclusive.add_argument('-n', '--no-deps', dest='no_deps', action='store_true', default=False,
                                       help="Don't download {0}s listed as dependencies.".format(galaxy_type))
        install_exclusive.add_argument('--force-with-deps', dest='force_with_deps', action='store_true', default=False,
                                       help="Force overwriting an existing {0} and its "
                                            "dependencies.".format(galaxy_type))
        if galaxy_type == 'collection':
            install_parser.add_argument('-p', '--collections-path', dest='collections_path',
                                        default=C.COLLECTIONS_PATHS[0],
                                        help='The path to the directory containing your collections.')
            install_parser.add_argument('-r', '--requirements-file', dest='requirements',
                                        help='A file containing a list of collections to be installed.')
            install_parser.add_argument('--pre', dest='allow_pre_release', action='store_true',