##### 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
```

validate_collection_name,
    validate_collection_path,
    verify_collections
)
from ansible.galaxy.login import GalaxyLogin
from ansible.galaxy.role import GalaxyRole
from ansible.galaxy.token import BasicAuthToken, GalaxyToken, KeycloakToken, NoTokenSentinel
from ansible.module_utils.ansible_release import __version__ as ansible_version
from ansible.module_utils.common.collections import is_iterable
from ansible.module_utils._text import to_bytes, to_native, to_text
from ansible.module_utils import six
from ansible.parsing.dataloader import DataLoader
from ansible.parsing.yaml.loader import AnsibleLoader
from ansible.playbook.role.requirement import RoleRequirement
from ansible.template import Templar
from ansible.utils.display import Display
from ansible.utils.plugin_docs import get_versioned_doclink
display = Display()
urlparse = six.moves.urllib.parse.urlparse
def _display_header(path, h1, h2, w1=10, w2=7):
    display.display('\n# {0}\n{1:{cwidth}} {2:{vwidth}}\n{3} {4}\n'.format(
        path,
        h1,
        h2,
        '-' * max([len(h1), w1]),  
        '-' * max([len(h2), w2]),
        cwidth=w1,
        vwidth=w2,
    ))
def _display_role(gr):