##### 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
```

---
            roles:
            # Same as v1 format just under the roles key
            collections:
            - namespace.collection
            - name: namespace.collection
              version: version identifier, multiple identifiers are separated by ','
              source: the URL or a predefined source name that relates to C.GALAXY_SERVER_LIST
        :param requirements_file: The path to the requirements file.
        :param allow_old_format: Will fail if a v1 requirements file is found and this is set to False.
        :return: a dict containing roles and collections to found in the requirements file.
        """
        requirements = {
            'roles': [],
            'collections': [],
        }
        b_requirements_file = to_bytes(requirements_file, errors='surrogate_or_strict')
        if not os.path.exists(b_requirements_file):
            raise AnsibleError("The requirements file '%s' does not exist." % to_native(requirements_file))
        display.vvv("Reading requirement file at '%s'" % requirements_file)
        with open(b_requirements_file, 'rb') as req_obj:
            try:
                file_requirements = yaml.safe_load(req_obj)
            except YAMLError as err:
                raise AnsibleError(
                    "Failed to parse the requirements yml at '%s' with the following error:\n%s"
                    % (to_native(requirements_file), to_native(err)))
        if file_requirements is None:
            raise AnsibleError("No requirements found in file '%s'" % to_native(requirements_file))
        def parse_role_req(requirement):