##### 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
```

login_parser = parser.add_parser('login', parents=parents,
                                         help="Login to api.github.com server in order to use ansible-galaxy role sub "
                                              "command such as 'import', 'delete', 'publish', and 'setup'")
        login_parser.set_defaults(func=self.execute_login)
        login_parser.add_argument('--github-token', dest='token', default=None,
                                  help='Identify with github token rather than username and password.')
    def add_info_options(self, parser, parents=None):
        info_parser = parser.add_parser('info', parents=parents, help='View more details about a specific role.')
        info_parser.set_defaults(func=self.execute_info)
        info_parser.add_argument('args', nargs='+', help='role', metavar='role_name[,version]')
    def add_verify_options(self, parser, parents=None):
        galaxy_type = 'collection'
        verify_parser = parser.add_parser('verify', parents=parents, help='Compare checksums with the collection(s) '
                                          'found on the server and the installed copy. This does not verify dependencies.')
        verify_parser.set_defaults(func=self.execute_verify)
        verify_parser.add_argument('args', metavar='{0}_name'.format(galaxy_type), nargs='*', help='The collection(s) name or '
                                   'path/url to a tar.gz collection artifact. This is mutually exclusive with --requirements-file.')
        verify_parser.add_argument('-i', '--ignore-errors', dest='ignore_errors', action='store_true', default=False,
                                   help='Ignore errors during verification and continue with the next specified collection.')
        verify_parser.add_argument('-r', '--requirements-file', dest='requirements',
                                   help='A file containing a list of collections to be verified.')
    def add_install_options(self, parser, parents=None):