##### 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
```

cmd_server = context.CLIARGS['api_server']
        cmd_token = GalaxyToken(token=context.CLIARGS['api_key'])
        if cmd_server:
            
            
            config_server = next((s for s in config_servers if s.name == cmd_server), None)
            if config_server:
                self.api_servers.append(config_server)
            else:
                self.api_servers.append(GalaxyAPI(self.galaxy, 'cmd_arg', cmd_server, token=cmd_token))
        else:
            self.api_servers = config_servers
        
        if len(self.api_servers) == 0:
            self.api_servers.append(GalaxyAPI(self.galaxy, 'default', C.GALAXY_SERVER, token=cmd_token))
        context.CLIARGS['func']()
    @property
    def api(self):
        return self.api_servers[0]
    def _parse_requirements_file(self, requirements_file, allow_old_format=True):
        """
        Parses an Ansible requirement.yml file and returns all the roles and/or collections defined in it. There are 2
        requirements file format:
            # v1 (roles only)
            - src: The source of the role, required if include is not set. Can be Galaxy role name, URL to a SCM repo or tarball.
              name: Downloads the role to the specified name, defaults to Galaxy name from Galaxy or name of repo if src is a URL.
              scm: If src is a URL, specify the SCM. Only git or hd are supported and defaults ot git.
              version: The version of the role to download. Can also be tag, commit, or branch name and defaults to master.
              include: Path to additional requirements.yml files.
            # v2 (roles and collections)