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##### SUMMARY
exception `ERROR! Unexpected Exception, this is probably a bug: 'NoneType' object has no attribute 'keys'` if requirements.yml is empty

2.8.x works as expected.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
galaxy

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
2.9.4
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->


##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml
# empty requirements.yml
---
```
run
```
 ansible-galaxy install -r requirements.yml -vvv
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->

ansible 2.8
```
 $ ansible-galaxy install -r requirements.yml     
ERROR! No roles found in file: requirements.yml
```

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->
ansible 2.9

```
ERROR! Unexpected Exception, this is probably a bug: 'NoneType' object has no attribute 'keys'
the full traceback was:

Traceback (most recent call last):
  File "/home/rmoser/.local/share/virtualenvs/ansible-puzzle-SlJRgz1S/bin/ansible-galaxy", line 123, in <module>
    exit_code = cli.run()
  File "/home/rmoser/.local/share/virtualenvs/ansible-puzzle-SlJRgz1S/lib/python3.6/site-packages/ansible/cli/galaxy.py", line 375, in run
    context.CLIARGS['func']()
  File "/home/rmoser/.local/share/virtualenvs/ansible-puzzle-SlJRgz1S/lib/python3.6/site-packages/ansible/cli/galaxy.py", line 857, in execute_install
    roles_left = self._parse_requirements_file(role_file)['roles']
  File "/home/rmoser/.local/share/virtualenvs/ansible-puzzle-SlJRgz1S/lib/python3.6/site-packages/ansible/cli/galaxy.py", line 461, in _parse_requirements_file
    extra_keys = set(file_requirements.keys()).difference(set(['roles', 'collections']))
AttributeError: 'NoneType' object has no attribute 'keys'
```

name = collection_input
                    else:
                        name, dummy, requirement = collection_input.partition(':')
                    requirements.append((name, requirement or '*', None))
            output_path = GalaxyCLI._resolve_path(output_path)
            collections_path = C.COLLECTIONS_PATHS
            if len([p for p in collections_path if p.startswith(output_path)]) == 0:
                display.warning("The specified collections path '%s' is not part of the configured Ansible "
                                "collections paths '%s'. The installed collection won't be picked up in an Ansible "
                                "run." % (to_text(output_path), to_text(":".join(collections_path))))
            output_path = validate_collection_path(output_path)
            b_output_path = to_bytes(output_path, errors='surrogate_or_strict')
            if not os.path.exists(b_output_path):
                os.makedirs(b_output_path)
            install_collections(requirements, output_path, self.api_servers, (not ignore_certs), ignore_errors,
                                no_deps, force, force_deps)
            return 0
        role_file = context.CLIARGS['role_file']
        if not context.CLIARGS['args'] and role_file is None:
            
            raise AnsibleOptionsError("- you must specify a user/role name or a roles file")
        no_deps = context.CLIARGS['no_deps']
        force_deps = context.CLIARGS['force_with_deps']
        force = context.CLIARGS['force'] or force_deps
        roles_left = []
        if role_file:
            if not (role_file.endswith('.yaml') or role_file.endswith('.yml')):
                raise AnsibleError("Invalid role requirements file, it must end with a .yml or .yaml extension")
            roles_left = self._parse_requirements_file(role_file)['roles']