##### 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
```

github.add_argument('github_repo', help='GitHub repository')
        offline = opt_help.argparse.ArgumentParser(add_help=False)
        offline.add_argument('--offline', dest='offline', default=False, action='store_true',
                             help="Don't query the galaxy API when creating roles")
        default_roles_path = C.config.get_configuration_definition('DEFAULT_ROLES_PATH').get('default', '')
        roles_path = opt_help.argparse.ArgumentParser(add_help=False)
        roles_path.add_argument('-p', '--roles-path', dest='roles_path', type=opt_help.unfrack_path(pathsep=True),
                                default=C.DEFAULT_ROLES_PATH, action=opt_help.PrependListAction,
                                help='The path to the directory containing your roles. The default is the first '
                                     'writable one configured via DEFAULT_ROLES_PATH: %s ' % default_roles_path)
        collections_path = opt_help.argparse.ArgumentParser(add_help=False)
        collections_path.add_argument('-p', '--collection-path', dest='collections_path', type=opt_help.unfrack_path(pathsep=True),
                                      default=C.COLLECTIONS_PATHS, action=opt_help.PrependListAction,
                                      help="One or more directories to search for collections in addition "
                                      "to the default COLLECTIONS_PATHS. Separate multiple paths "
                                      "with '{0}'.".format(os.path.pathsep))
        
        type_parser = self.parser.add_subparsers(metavar='TYPE', dest='type')
        type_parser.required = True
        
        collection = type_parser.add_parser('collection', help='Manage an Ansible Galaxy collection.')
        collection_parser = collection.add_subparsers(metavar='COLLECTION_ACTION', dest='action')
        collection_parser.required = True
        self.add_download_options(collection_parser, parents=[common])
        self.add_init_options(collection_parser, parents=[common, force])
        self.add_build_options(collection_parser, parents=[common, force])
        self.add_publish_options(collection_parser, parents=[common])
        self.add_install_options(collection_parser, parents=[common, force])
        self.add_list_options(collection_parser, parents=[common, collections_path])
        self.add_verify_options(collection_parser, parents=[common, collections_path])