##### 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.warning(u"- %s was NOT installed successfully: %s " % (role.name, to_text(e)))
                self.exit_without_ignore()
                continue
            
            if not no_deps and installed:
                if not role.metadata:
                    display.warning("Meta file %s is empty. Skipping dependencies." % role.path)
                else:
                    role_dependencies = (role.metadata.get('dependencies') or []) + role.requirements
                    for dep in role_dependencies:
                        display.debug('Installing dep %s' % dep)
                        dep_req = RoleRequirement()
                        dep_info = dep_req.role_yaml_parse(dep)
                        dep_role = GalaxyRole(self.galaxy, self.api, **dep_info)
                        if '.' not in dep_role.name and '.' not in dep_role.src and dep_role.scm is None:
                            
                            
                            continue
                        if dep_role.install_info is None:
                            if dep_role not in roles_left:
                                display.display('- adding dependency: %s' % to_text(dep_role))
                                roles_left.append(dep_role)
                            else:
                                display.display('- dependency %s already pending installation.' % dep_role.name)
                        else:
                            if dep_role.install_info['version'] != dep_role.version:
                                if force_deps:
                                    display.display('- changing dependant role %s from %s to %s' %
                                                    (dep_role.name, dep_role.install_info['version'], dep_role.version or "unspecified"))
                                    dep_role.remove()