<!--- 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'
```

""" Setup an integration from Github or Travis for Ansible Galaxy roles"""
        if context.CLIARGS['setup_list']:
            
            secrets = self.api.list_secrets()
            if len(secrets) == 0:
                
                display.display("No integrations found.")
                return 0
            display.display(u'\n' + "ID         Source     Repo", color=C.COLOR_OK)
            display.display("---------- ---------- ----------", color=C.COLOR_OK)
            for secret in secrets:
                display.display("%-10s %-10s %s/%s" % (secret['id'], secret['source'], secret['github_user'],
                                                       secret['github_repo']), color=C.COLOR_OK)
            return 0
        if context.CLIARGS['remove_id']:
            
            self.api.remove_secret(context.CLIARGS['remove_id'])
            display.display("Secret removed. Integrations using this secret will not longer work.", color=C.COLOR_OK)
            return 0
        source = context.CLIARGS['source']
        github_user = context.CLIARGS['github_user']
        github_repo = context.CLIARGS['github_repo']
        secret = context.CLIARGS['secret']
        resp = self.api.add_secret(source, github_user, github_repo, secret)
        display.display("Added integration for %s %s/%s" % (resp['source'], resp['github_user'], resp['github_repo']))
        return 0
    def execute_delete(self):