##### 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
```

filename, ext = os.path.splitext(f)
                if any(r.match(os.path.join(rel_root, f)) for r in skeleton_ignore_re):
                    continue
                if galaxy_type == 'collection' and own_skeleton and rel_root == '.' and f == 'galaxy.yml.j2':
                    
                    
                    
                    
                    template_data = inject_data.copy()
                    template_data['name'] = template_data.pop('collection_name')
                    meta_value = GalaxyCLI._get_skeleton_galaxy_yml(os.path.join(root, rel_root, f), template_data)
                    b_dest_file = to_bytes(os.path.join(obj_path, rel_root, filename), errors='surrogate_or_strict')
                    with open(b_dest_file, 'wb') as galaxy_obj:
                        galaxy_obj.write(to_bytes(meta_value, errors='surrogate_or_strict'))
                elif ext == ".j2" and not in_templates_dir:
                    src_template = os.path.join(root, f)
                    dest_file = os.path.join(obj_path, rel_root, filename)
                    template_data = to_text(loader._get_file_contents(src_template)[0], errors='surrogate_or_strict')
                    b_rendered = to_bytes(templar.template(template_data), errors='surrogate_or_strict')
                    with open(dest_file, 'wb') as df:
                        df.write(b_rendered)
                else:
                    f_rel_path = os.path.relpath(os.path.join(root, f), obj_skeleton)
                    shutil.copyfile(os.path.join(root, f), os.path.join(obj_path, f_rel_path))
            for d in dirs:
                b_dir_path = to_bytes(os.path.join(obj_path, rel_root, d), errors='surrogate_or_strict')
                if not os.path.exists(b_dir_path):
                    os.makedirs(b_dir_path)
        display.display("- %s %s was created successfully" % (galaxy_type.title(), obj_name))
    def execute_info(self):