##### 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
```

continue
            if not os.path.isdir(collection_path):
                warnings.append("- the configured path {0}, exists, but it is not a directory.".format(collection_path))
                continue
            path_found = True
            if collection_name:
                
                validate_collection_name(collection_name)
                namespace, collection = collection_name.split('.')
                collection_path = validate_collection_path(collection_path)
                b_collection_path = to_bytes(os.path.join(collection_path, namespace, collection), errors='surrogate_or_strict')
                if not os.path.exists(b_collection_path):
                    warnings.append("- unable to find {0} in collection paths".format(collection_name))
                    continue
                if not os.path.isdir(collection_path):
                    warnings.append("- the configured path {0}, exists, but it is not a directory.".format(collection_path))
                    continue
                collection_found = True
                collection = CollectionRequirement.from_path(b_collection_path, False)
                fqcn_width, version_width = _get_collection_widths(collection)
                _display_header(collection_path, 'Collection', 'Version', fqcn_width, version_width)
                _display_collection(collection, fqcn_width, version_width)
            else:
                
                collection_path = validate_collection_path(path)
                if os.path.isdir(collection_path):
                    display.vvv("Searching {0} for collections".format(collection_path))
                    collections = find_existing_collections(collection_path)
                else: