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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'
```

galaxy_type = 'collection' if parser.metavar == 'COLLECTION_ACTION' else 'role'
        args_kwargs = {}
        if galaxy_type == 'collection':
            args_kwargs['help'] = 'The collection(s) name or path/url to a tar.gz collection artifact. This is ' \
                                  'mutually exclusive with --requirements-file.'
            ignore_errors_help = 'Ignore errors during installation and continue with the next specified ' \
                                 'collection. This will not ignore dependency conflict errors.'
        else:
            args_kwargs['help'] = 'Role name, URL or tar file'
            ignore_errors_help = 'Ignore errors and continue with the next specified role.'
        install_parser = parser.add_parser('install', parents=parents,
                                           help='Install {0}(s) from file(s), URL(s) or Ansible '
                                                'Galaxy'.format(galaxy_type))
        install_parser.set_defaults(func=self.execute_install)
        install_parser.add_argument('args', metavar='{0}_name'.format(galaxy_type), nargs='*', **args_kwargs)
        install_parser.add_argument('-i', '--ignore-errors', dest='ignore_errors', action='store_true', default=False,
                                    help=ignore_errors_help)
        install_exclusive = install_parser.add_mutually_exclusive_group()
        install_exclusive.add_argument('-n', '--no-deps', dest='no_deps', action='store_true', default=False,
                                       help="Don't download {0}s listed as dependencies.".format(galaxy_type))
        install_exclusive.add_argument('--force-with-deps', dest='force_with_deps', action='store_true', default=False,
                                       help="Force overwriting an existing {0} and its "
                                            "dependencies.".format(galaxy_type))
        if galaxy_type == 'collection':
            install_parser.add_argument('-p', '--collections-path', dest='collections_path',
                                        default=C.COLLECTIONS_PATHS[0],
                                        help='The path to the directory containing your collections.')
            install_parser.add_argument('-r', '--requirements-file', dest='requirements',