##### 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
```

remove_parser = parser.add_parser('remove', parents=parents, help='Delete roles from roles_path.')
        remove_parser.set_defaults(func=self.execute_remove)
        remove_parser.add_argument('args', help='Role(s)', metavar='role', nargs='+')
    def add_delete_options(self, parser, parents=None):
        delete_parser = parser.add_parser('delete', parents=parents,
                                          help='Removes the role from Galaxy. It does not remove or alter the actual '
                                               'GitHub repository.')
        delete_parser.set_defaults(func=self.execute_delete)
    def add_list_options(self, parser, parents=None):
        galaxy_type = 'role'
        if parser.metavar == 'COLLECTION_ACTION':
            galaxy_type = 'collection'
        list_parser = parser.add_parser('list', parents=parents,
                                        help='Show the name and version of each {0} installed in the {0}s_path.'.format(galaxy_type))
        list_parser.set_defaults(func=self.execute_list)
        list_parser.add_argument(galaxy_type, help=galaxy_type.capitalize(), nargs='?', metavar=galaxy_type)
    def add_search_options(self, parser, parents=None):
        search_parser = parser.add_parser('search', parents=parents,
                                          help='Search the Galaxy database by tags, platforms, author and multiple '
                                               'keywords.')
        search_parser.set_defaults(func=self.execute_search)
        search_parser.add_argument('--platforms', dest='platforms', help='list of OS platforms to filter by')
        search_parser.add_argument('--galaxy-tags', dest='galaxy_tags', help='list of galaxy tags to filter by')
        search_parser.add_argument('--author', dest='author', help='GitHub username')
        search_parser.add_argument('args', help='Search terms', metavar='searchterm', nargs='*')
    def add_import_options(self, parser, parents=None):