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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'
```

''' searches for roles on the Ansible Galaxy server'''
        page_size = 1000
        search = None
        if context.CLIARGS['args']:
            search = '+'.join(context.CLIARGS['args'])
        if not search and not context.CLIARGS['platforms'] and not context.CLIARGS['galaxy_tags'] and not context.CLIARGS['author']:
            raise AnsibleError("Invalid query. At least one search term, platform, galaxy tag or author must be provided.")
        response = self.api.search_roles(search, platforms=context.CLIARGS['platforms'],
                                         tags=context.CLIARGS['galaxy_tags'], author=context.CLIARGS['author'], page_size=page_size)
        if response['count'] == 0:
            display.display("No roles match your search.", color=C.COLOR_ERROR)
            return True
        data = [u'']
        if response['count'] > page_size:
            data.append(u"Found %d roles matching your search. Showing first %s." % (response['count'], page_size))
        else:
            data.append(u"Found %d roles matching your search:" % response['count'])
        max_len = []
        for role in response['results']:
            max_len.append(len(role['username'] + '.' + role['name']))
        name_len = max(max_len)
        format_str = u" %%-%ds %%s" % name_len
        data.append(u'')