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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'
```

setup_parser = parser.add_parser('setup', parents=parents,
                                         help='Manage the integration between Galaxy and the given source.')
        setup_parser.set_defaults(func=self.execute_setup)
        setup_parser.add_argument('--remove', dest='remove_id', default=None,
                                  help='Remove the integration matching the provided ID value. Use --list to see '
                                       'ID values.')
        setup_parser.add_argument('--list', dest="setup_list", action='store_true', default=False,
                                  help='List all of your integrations.')
        setup_parser.add_argument('source', help='Source')
        setup_parser.add_argument('github_user', help='GitHub username')
        setup_parser.add_argument('github_repo', help='GitHub repository')
        setup_parser.add_argument('secret', help='Secret')
    def add_login_options(self, parser, parents=None):
        login_parser = parser.add_parser('login', parents=parents,
                                         help="Login to api.github.com server in order to use ansible-galaxy role sub "
                                              "command such as 'import', 'delete', 'publish', and 'setup'")
        login_parser.set_defaults(func=self.execute_login)
        login_parser.add_argument('--github-token', dest='token', default=None,
                                  help='Identify with github token rather than username and password.')
    def add_info_options(self, parser, parents=None):
        info_parser = parser.add_parser('info', parents=parents, help='View more details about a specific role.')
        info_parser.set_defaults(func=self.execute_info)
        info_parser.add_argument('args', nargs='+', help='role', metavar='role_name[,version]')
    def add_install_options(self, parser, parents=None):