##### 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
```

role = type_parser.add_parser('role', help='Manage an Ansible Galaxy role.')
        role_parser = role.add_subparsers(metavar='ROLE_ACTION', dest='action')
        role_parser.required = True
        self.add_init_options(role_parser, parents=[common, force, offline])
        self.add_remove_options(role_parser, parents=[common, roles_path])
        self.add_delete_options(role_parser, parents=[common, github])
        self.add_list_options(role_parser, parents=[common, roles_path])
        self.add_search_options(role_parser, parents=[common])
        self.add_import_options(role_parser, parents=[common, github])
        self.add_setup_options(role_parser, parents=[common, roles_path])
        self.add_login_options(role_parser, parents=[common])
        self.add_info_options(role_parser, parents=[common, roles_path, offline])
        self.add_install_options(role_parser, parents=[common, force, roles_path])
    def add_download_options(self, parser, parents=None):
        download_parser = parser.add_parser('download', parents=parents,
                                            help='Download collections and their dependencies as a tarball for an '
                                                 'offline install.')
        download_parser.set_defaults(func=self.execute_download)
        download_parser.add_argument('args', help='Collection(s)', metavar='collection', nargs='*')
        download_parser.add_argument('-n', '--no-deps', dest='no_deps', action='store_true', default=False,
                                     help="Don't download collection(s) listed as dependencies.")
        download_parser.add_argument('-p', '--download-path', dest='download_path',
                                     default='./collections',
                                     help='The directory to download the collections to.')
        download_parser.add_argument('-r', '--requirements-file', dest='requirements',
                                     help='A file containing a list of collections to be downloaded.')
        download_parser.add_argument('--pre', dest='allow_pre_release', action='store_true',
                                     help='Include pre-release versions. Semantic versioning pre-releases are ignored by default')
    def add_init_options(self, parser, parents=None):