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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'
```

config_servers = []
        
        server_list = [s for s in C.GALAXY_SERVER_LIST or [] if s]
        for server_key in server_list:
            
            
            config_dict = dict((k, server_config_def(server_key, k, req)) for k, req in server_def)
            defs = AnsibleLoader(yaml.safe_dump(config_dict)).get_single_data()
            C.config.initialize_plugin_configuration_definitions('galaxy_server', server_key, defs)
            server_options = C.config.get_plugin_options('galaxy_server', server_key)
            
            
            auth_url = server_options.pop('auth_url', None)
            token_val = server_options['token'] or NoTokenSentinel
            username = server_options['username']
            
            server_options['token'] = None
            if username:
                server_options['token'] = BasicAuthToken(username,
                                                         server_options['password'])
            else:
                if token_val:
                    if auth_url:
                        server_options['token'] = KeycloakToken(access_token=token_val,
                                                                auth_url=auth_url,
                                                                validate_certs=not context.CLIARGS['ignore_certs'])
                    else:
                        
                        server_options['token'] = GalaxyToken(token=token_val)
            config_servers.append(GalaxyAPI(self.galaxy, server_key, **server_options))