<!--- Verify first that your issue is not already reported on GitHub -->
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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'
```

''' create an options parser for bin/ansible '''
        super(GalaxyCLI, self).init_parser(
            desc="Perform various Role and Collection related operations.",
        )
        
        common = opt_help.argparse.ArgumentParser(add_help=False)
        common.add_argument('-s', '--server', dest='api_server', help='The Galaxy API server URL')
        common.add_argument('--token', '--api-key', dest='api_key',
                            help='The Ansible Galaxy API key which can be found at '
                                 'https://galaxy.ansible.com/me/preferences. You can also use ansible-galaxy login to '
                                 'retrieve this key or set the token for the GALAXY_SERVER_LIST entry.')
        common.add_argument('-c', '--ignore-certs', action='store_true', dest='ignore_certs',
                            default=C.GALAXY_IGNORE_CERTS, help='Ignore SSL certificate validation errors.')
        opt_help.add_verbosity_options(common)
        force = opt_help.argparse.ArgumentParser(add_help=False)
        force.add_argument('-f', '--force', dest='force', action='store_true', default=False,
                           help='Force overwriting an existing role or collection')
        github = opt_help.argparse.ArgumentParser(add_help=False)
        github.add_argument('github_user', help='GitHub username')
        github.add_argument('github_repo', help='GitHub repository')
        offline = opt_help.argparse.ArgumentParser(add_help=False)
        offline.add_argument('--offline', dest='offline', default=False, action='store_true',
                             help="Don't query the galaxy API when creating roles")
        default_roles_path = C.config.get_configuration_definition('DEFAULT_ROLES_PATH').get('default', '')
        roles_path = opt_help.argparse.ArgumentParser(add_help=False)