##### 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'
        init_parser = parser.add_parser('init', parents=parents,
                                        help='Initialize new {0} with the base structure of a '
                                             '{0}.'.format(galaxy_type))
        init_parser.set_defaults(func=self.execute_init)
        init_parser.add_argument('--init-path', dest='init_path', default='./',
                                 help='The path in which the skeleton {0} will be created. The default is the '
                                      'current working directory.'.format(galaxy_type))
        init_parser.add_argument('--{0}-skeleton'.format(galaxy_type), dest='{0}_skeleton'.format(galaxy_type),
                                 default=C.GALAXY_ROLE_SKELETON,
                                 help='The path to a {0} skeleton that the new {0} should be based '
                                      'upon.'.format(galaxy_type))
        obj_name_kwargs = {}
        if galaxy_type == 'collection':
            obj_name_kwargs['type'] = validate_collection_name
        init_parser.add_argument('{0}_name'.format(galaxy_type), help='{0} name'.format(galaxy_type.capitalize()),
                                 **obj_name_kwargs)
        if galaxy_type == 'role':
            init_parser.add_argument('--type', dest='role_type', action='store', default='default',
                                     help="Initialize using an alternate role type. Valid types include: 'container', "
                                          "'apb' and 'network'.")
    def add_remove_options(self, parser, parents=None):