##### 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
```

display.display("WARNING: More than one Galaxy role associated with Github repo %s/%s." % (github_user, github_repo),
                                color='yellow')
                display.display("The following Galaxy roles are being updated:" + u'\n', color=C.COLOR_CHANGED)
                for t in task:
                    display.display('%s.%s' % (t['summary_fields']['role']['namespace'], t['summary_fields']['role']['name']), color=C.COLOR_CHANGED)
                display.display(u'\nTo properly namespace this role, remove each of the above and re-import %s/%s from scratch' % (github_user, github_repo),
                                color=C.COLOR_CHANGED)
                return 0
            
            display.display("Successfully submitted import request %d" % task[0]['id'])
            if not context.CLIARGS['wait']:
                display.display("Role name: %s" % task[0]['summary_fields']['role']['name'])
                display.display("Repo: %s/%s" % (task[0]['github_user'], task[0]['github_repo']))
        if context.CLIARGS['check_status'] or context.CLIARGS['wait']:
            
            msg_list = []
            finished = False
            while not finished:
                task = self.api.get_import_task(task_id=task[0]['id'])
                for msg in task[0]['summary_fields']['task_messages']:
                    if msg['id'] not in msg_list:
                        display.display(msg['message_text'], color=colors[msg['message_type']])
                        msg_list.append(msg['id'])
                if task[0]['state'] in ['SUCCESS', 'FAILED']:
                    finished = True
                else:
                    time.sleep(10)
        return 0
    def execute_setup(self):