##### 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
```

install_info = gr.install_info
    version = None
    if install_info:
        version = install_info.get("version", None)
    if not version:
        version = "(unknown version)"
    display.display("- %s, %s" % (gr.name, version))
def _display_collection(collection, cwidth=10, vwidth=7, min_cwidth=10, min_vwidth=7):
    display.display('{fqcn:{cwidth}} {version:{vwidth}}'.format(
        fqcn=to_text(collection),
        version=collection.latest_version,
        cwidth=max(cwidth, min_cwidth),  
        vwidth=max(vwidth, min_vwidth)
    ))
def _get_collection_widths(collections):
    if is_iterable(collections):
        fqcn_set = set(to_text(c) for c in collections)
        version_set = set(to_text(c.latest_version) for c in collections)
    else:
        fqcn_set = set([to_text(collections)])
        version_set = set([collections.latest_version])
    fqcn_length = len(max(fqcn_set, key=len))
    version_length = len(max(version_set, key=len))
    return fqcn_length, version_length
class GalaxyCLI(CLI):
    '''command to manage Ansible roles in shared repositories, the default of which is Ansible Galaxy *https://galaxy.ansible.com*.'''
    SKIP_INFO_KEYS = ("name", "description", "readme_html", "related", "summary_fields", "average_aw_composite", "average_aw_score", "url")
    def __init__(self, args):