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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'
```

cmd_server = context.CLIARGS['api_server']
        cmd_token = GalaxyToken(token=context.CLIARGS['api_key'])
        if cmd_server:
            
            
            config_server = next((s for s in config_servers if s.name == cmd_server), None)
            if config_server:
                self.api_servers.append(config_server)
            else:
                self.api_servers.append(GalaxyAPI(self.galaxy, 'cmd_arg', cmd_server, token=cmd_token))
        else:
            self.api_servers = config_servers
        
        if len(self.api_servers) == 0:
            self.api_servers.append(GalaxyAPI(self.galaxy, 'default', C.GALAXY_SERVER, token=cmd_token))
        context.CLIARGS['func']()
    @property
    def api(self):
        return self.api_servers[0]
    def _parse_requirements_file(self, requirements_file, allow_old_format=True):
        """
        Parses an Ansible requirement.yml file and returns all the roles and/or collections defined in it. There are 2
        requirements file format:
            # v1 (roles only)
            - src: The source of the role, required if include is not set. Can be Galaxy role name, URL to a SCM repo or tarball.
              name: Downloads the role to the specified name, defaults to Galaxy name from Galaxy or name of repo if src is a URL.
              scm: If src is a URL, specify the SCM. Only git or hd are supported and defaults ot git.
              version: The version of the role to download. Can also be tag, commit, or branch name and defaults to master.
              include: Path to additional requirements.yml files.
            # v2 (roles and collections)