<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
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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'
```

display.display("WARNING: More than one Galaxy role associated with Github repo %s/%s." % (github_user, github_repo),
                                color='yellow')
                display.display("The following Galaxy roles are being updated:" + u'\n', color=C.COLOR_CHANGED)
                for t in task:
                    display.display('%s.%s' % (t['summary_fields']['role']['namespace'], t['summary_fields']['role']['name']), color=C.COLOR_CHANGED)
                display.display(u'\nTo properly namespace this role, remove each of the above and re-import %s/%s from scratch' % (github_user, github_repo),
                                color=C.COLOR_CHANGED)
                return 0
            
            display.display("Successfully submitted import request %d" % task[0]['id'])
            if not context.CLIARGS['wait']:
                display.display("Role name: %s" % task[0]['summary_fields']['role']['name'])
                display.display("Repo: %s/%s" % (task[0]['github_user'], task[0]['github_repo']))
        if context.CLIARGS['check_status'] or context.CLIARGS['wait']:
            
            msg_list = []
            finished = False
            while not finished:
                task = self.api.get_import_task(task_id=task[0]['id'])
                for msg in task[0]['summary_fields']['task_messages']:
                    if msg['id'] not in msg_list:
                        display.display(msg['message_text'], color=colors[msg['message_type']])
                        msg_list.append(msg['id'])
                if task[0]['state'] in ['SUCCESS', 'FAILED']:
                    finished = True
                else:
                    time.sleep(10)
        return 0
    def execute_setup(self):