##### 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
```

import_parser = parser.add_parser('import', parents=parents, help='Import a role into a galaxy server')
        import_parser.set_defaults(func=self.execute_import)
        import_parser.add_argument('--no-wait', dest='wait', action='store_false', default=True,
                                   help="Don't wait for import results.")
        import_parser.add_argument('--branch', dest='reference',
                                   help='The name of a branch to import. Defaults to the repository\'s default branch '
                                        '(usually master)')
        import_parser.add_argument('--role-name', dest='role_name',
                                   help='The name the role should have, if different than the repo name')
        import_parser.add_argument('--status', dest='check_status', action='store_true', default=False,
                                   help='Check the status of the most recent import request for given github_'
                                        'user/github_repo.')
    def add_setup_options(self, parser, parents=None):
        setup_parser = parser.add_parser('setup', parents=parents,
                                         help='Manage the integration between Galaxy and the given source.')
        setup_parser.set_defaults(func=self.execute_setup)
        setup_parser.add_argument('--remove', dest='remove_id', default=None,
                                  help='Remove the integration matching the provided ID value. Use --list to see '
                                       'ID values.')
        setup_parser.add_argument('--list', dest="setup_list", action='store_true', default=False,
                                  help='List all of your integrations.')
        setup_parser.add_argument('source', help='Source')
        setup_parser.add_argument('github_user', help='GitHub username')
        setup_parser.add_argument('github_repo', help='GitHub repository')
        setup_parser.add_argument('secret', help='Secret')
    def add_login_options(self, parser, parents=None):