##### 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
```

help='Include pre-release versions. Semantic versioning pre-releases are ignored by default')
        else:
            install_parser.add_argument('-r', '--role-file', dest='role_file',
                                        help='A file containing a list of roles to be imported.')
            install_parser.add_argument('-g', '--keep-scm-meta', dest='keep_scm_meta', action='store_true',
                                        default=False,
                                        help='Use tar instead of the scm archive option when packaging the role.')
    def add_build_options(self, parser, parents=None):
        build_parser = parser.add_parser('build', parents=parents,
                                         help='Build an Ansible collection artifact that can be publish to Ansible '
                                              'Galaxy.')
        build_parser.set_defaults(func=self.execute_build)
        build_parser.add_argument('args', metavar='collection', nargs='*', default=('.',),
                                  help='Path to the collection(s) directory to build. This should be the directory '
                                       'that contains the galaxy.yml file. The default is the current working '
                                       'directory.')
        build_parser.add_argument('--output-path', dest='output_path', default='./',
                                  help='The path in which the collection is built to. The default is the current '
                                       'working directory.')
    def add_publish_options(self, parser, parents=None):
        publish_parser = parser.add_parser('publish', parents=parents,
                                           help='Publish a collection artifact to Ansible Galaxy.')
        publish_parser.set_defaults(func=self.execute_publish)
        publish_parser.add_argument('args', metavar='collection_path',
                                    help='The path to the collection tarball to publish.')
        publish_parser.add_argument('--no-wait', dest='wait', action='store_false', default=True,
                                    help="Don't wait for import validation results.")
        publish_parser.add_argument('--import-timeout', dest='import_timeout', type=int, default=0,
                                    help="The time to wait for the collection import process to finish.")
    def post_process_args(self, options):