<!--- 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'
```

roles_path.add_argument('-p', '--roles-path', dest='roles_path', type=opt_help.unfrack_path(pathsep=True),
                                default=C.DEFAULT_ROLES_PATH, action=opt_help.PrependListAction,
                                help='The path to the directory containing your roles. The default is the first '
                                     'writable one configured via DEFAULT_ROLES_PATH: %s ' % default_roles_path)
        
        type_parser = self.parser.add_subparsers(metavar='TYPE', dest='type')
        type_parser.required = True
        
        collection = type_parser.add_parser('collection', help='Manage an Ansible Galaxy collection.')
        collection_parser = collection.add_subparsers(metavar='COLLECTION_ACTION', dest='action')
        collection_parser.required = True
        self.add_init_options(collection_parser, parents=[common, force])
        self.add_build_options(collection_parser, parents=[common, force])
        self.add_publish_options(collection_parser, parents=[common])
        self.add_install_options(collection_parser, parents=[common, force])
        
        role = type_parser.add_parser('role', help='Manage an Ansible Galaxy role.')
        role_parser = role.add_subparsers(metavar='ROLE_ACTION', dest='action')
        role_parser.required = True
        self.add_init_options(role_parser, parents=[common, force, offline])
        self.add_remove_options(role_parser, parents=[common, roles_path])
        self.add_delete_options(role_parser, parents=[common, github])
        self.add_list_options(role_parser, parents=[common, roles_path])
        self.add_search_options(role_parser, parents=[common])
        self.add_import_options(role_parser, parents=[common, github])
        self.add_setup_options(role_parser, parents=[common, roles_path])
        self.add_login_options(role_parser, parents=[common])
        self.add_info_options(role_parser, parents=[common, roles_path, offline])
        self.add_install_options(role_parser, parents=[common, force, roles_path])
    def add_init_options(self, parser, parents=None):