##### 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
```

if len(args) > 1 and args[1] not in ['-h', '--help', '--version'] and 'role' not in args and 'collection' not in args:
            
            
            idx = 2 if args[1].startswith('-v') else 1
            args.insert(idx, 'role')
        self.api_servers = []
        self.galaxy = None
        super(GalaxyCLI, self).__init__(args)
    def init_parser(self):
        ''' create an options parser for bin/ansible '''
        super(GalaxyCLI, self).init_parser(
            desc="Perform various Role and Collection related operations.",
        )
        
        common = opt_help.argparse.ArgumentParser(add_help=False)
        common.add_argument('-s', '--server', dest='api_server', help='The Galaxy API server URL')
        common.add_argument('--token', '--api-key', dest='api_key',
                            help='The Ansible Galaxy API key which can be found at '
                                 'https://galaxy.ansible.com/me/preferences. You can also use ansible-galaxy login to '
                                 'retrieve this key or set the token for the GALAXY_SERVER_LIST entry.')
        common.add_argument('-c', '--ignore-certs', action='store_true', dest='ignore_certs',
                            default=C.GALAXY_IGNORE_CERTS, help='Ignore SSL certificate validation errors.')
        opt_help.add_verbosity_options(common)
        force = opt_help.argparse.ArgumentParser(add_help=False)
        force.add_argument('-f', '--force', dest='force', action='store_true', default=False,
                           help='Force overwriting an existing role or collection')
        github = opt_help.argparse.ArgumentParser(add_help=False)
        github.add_argument('github_user', help='GitHub username')