##### 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
```

""" Setup an integration from Github or Travis for Ansible Galaxy roles"""
        if context.CLIARGS['setup_list']:
            
            secrets = self.api.list_secrets()
            if len(secrets) == 0:
                
                display.display("No integrations found.")
                return 0
            display.display(u'\n' + "ID         Source     Repo", color=C.COLOR_OK)
            display.display("---------- ---------- ----------", color=C.COLOR_OK)
            for secret in secrets:
                display.display("%-10s %-10s %s/%s" % (secret['id'], secret['source'], secret['github_user'],
                                                       secret['github_repo']), color=C.COLOR_OK)
            return 0
        if context.CLIARGS['remove_id']:
            
            self.api.remove_secret(context.CLIARGS['remove_id'])
            display.display("Secret removed. Integrations using this secret will not longer work.", color=C.COLOR_OK)
            return 0
        source = context.CLIARGS['source']
        github_user = context.CLIARGS['github_user']
        github_repo = context.CLIARGS['github_repo']
        secret = context.CLIARGS['secret']
        resp = self.api.add_secret(source, github_user, github_repo, secret)
        display.display("Added integration for %s %s/%s" % (resp['source'], resp['github_user'], resp['github_repo']))
        return 0
    def execute_delete(self):