<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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'
```

publish_collection,
    validate_collection_name,
    validate_collection_path,
)
from ansible.galaxy.login import GalaxyLogin
from ansible.galaxy.role import GalaxyRole
from ansible.galaxy.token import BasicAuthToken, GalaxyToken, KeycloakToken, NoTokenSentinel
from ansible.module_utils.ansible_release import __version__ as ansible_version
from ansible.module_utils._text import to_bytes, to_native, to_text
from ansible.module_utils import six
from ansible.parsing.yaml.loader import AnsibleLoader
from ansible.playbook.role.requirement import RoleRequirement
from ansible.utils.display import Display
from ansible.utils.plugin_docs import get_versioned_doclink
display = Display()
urlparse = six.moves.urllib.parse.urlparse
class GalaxyCLI(CLI):
    '''command to manage Ansible roles in shared repositories, the default of which is Ansible Galaxy *https://galaxy.ansible.com*.'''
    SKIP_INFO_KEYS = ("name", "description", "readme_html", "related", "summary_fields", "average_aw_composite", "average_aw_score", "url")
    def __init__(self, args):
        
        if len(args) > 1 and args[1] not in ['-h', '--help', '--version'] and 'role' not in args and 'collection' not in args:
            
            
            idx = 2 if args[1].startswith('-v') else 1
            args.insert(idx, 'role')
        self.api_servers = []
        self.galaxy = None
        super(GalaxyCLI, self).__init__(args)
    def init_parser(self):