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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'
        init_parser = parser.add_parser('init', parents=parents,
                                        help='Initialize new {0} with the base structure of a '
                                             '{0}.'.format(galaxy_type))
        init_parser.set_defaults(func=self.execute_init)
        init_parser.add_argument('--init-path', dest='init_path', default='./',
                                 help='The path in which the skeleton {0} will be created. The default is the '
                                      'current working directory.'.format(galaxy_type))
        init_parser.add_argument('--{0}-skeleton'.format(galaxy_type), dest='{0}_skeleton'.format(galaxy_type),
                                 default=C.GALAXY_ROLE_SKELETON,
                                 help='The path to a {0} skeleton that the new {0} should be based '
                                      'upon.'.format(galaxy_type))
        obj_name_kwargs = {}
        if galaxy_type == 'collection':
            obj_name_kwargs['type'] = validate_collection_name
        init_parser.add_argument('{0}_name'.format(galaxy_type), help='{0} name'.format(galaxy_type.capitalize()),
                                 **obj_name_kwargs)
        if galaxy_type == 'role':
            init_parser.add_argument('--type', dest='role_type', action='store', default='default',
                                     help="Initialize using an alternate role type. Valid types include: 'container', "
                                          "'apb' and 'network'.")
    def add_remove_options(self, parser, parents=None):