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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'
```

license='license (GPL-2.0-or-later, MIT, etc)',
                role_name=obj_name,
                role_type=context.CLIARGS['role_type'],
                issue_tracker_url='http://example.com/issue/tracker',
                repository_url='http://example.com/repository',
                documentation_url='http://docs.example.com',
                homepage_url='http://example.com',
                min_ansible_version=ansible_version[:3],  
            ))
            obj_path = os.path.join(init_path, obj_name)
        elif galaxy_type == 'collection':
            namespace, collection_name = obj_name.split('.', 1)
            inject_data.update(dict(
                namespace=namespace,
                collection_name=collection_name,
                version='1.0.0',
                readme='README.md',
                authors=['your name <example@domain.com>'],
                license=['GPL-2.0-or-later'],
                repository='http://example.com/repository',
                documentation='http://docs.example.com',
                homepage='http://example.com',
                issues='http://example.com/issue/tracker',
                build_ignore=[],
            ))
            obj_path = os.path.join(init_path, namespace, collection_name)
        b_obj_path = to_bytes(obj_path, errors='surrogate_or_strict')
        if os.path.exists(b_obj_path):
            if os.path.isfile(obj_path):
                raise AnsibleError("- the path %s already exists, but is a file - aborting" % to_native(obj_path))