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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'
```

help='A file containing a list of collections to be installed.')
        else:
            install_parser.add_argument('-r', '--role-file', dest='role_file',
                                        help='A file containing a list of roles to be imported.')
            install_parser.add_argument('-g', '--keep-scm-meta', dest='keep_scm_meta', action='store_true',
                                        default=False,
                                        help='Use tar instead of the scm archive option when packaging the role.')
    def add_build_options(self, parser, parents=None):
        build_parser = parser.add_parser('build', parents=parents,
                                         help='Build an Ansible collection artifact that can be publish to Ansible '
                                              'Galaxy.')
        build_parser.set_defaults(func=self.execute_build)
        build_parser.add_argument('args', metavar='collection', nargs='*', default=('.',),
                                  help='Path to the collection(s) directory to build. This should be the directory '
                                       'that contains the galaxy.yml file. The default is the current working '
                                       'directory.')
        build_parser.add_argument('--output-path', dest='output_path', default='./',
                                  help='The path in which the collection is built to. The default is the current '
                                       'working directory.')
    def add_publish_options(self, parser, parents=None):
        publish_parser = parser.add_parser('publish', parents=parents,
                                           help='Publish a collection artifact to Ansible Galaxy.')
        publish_parser.set_defaults(func=self.execute_publish)
        publish_parser.add_argument('args', metavar='collection_path',
                                    help='The path to the collection tarball to publish.')
        publish_parser.add_argument('--no-wait', dest='wait', action='store_false', default=True,
                                    help="Don't wait for import validation results.")
        publish_parser.add_argument('--import-timeout', dest='import_timeout', type=int, default=0,
                                    help="The time to wait for the collection import process to finish.")
    def post_process_args(self, options):