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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'
```

roles_left.append(dep_role)
                                else:
                                    display.warning('- dependency %s (%s) from role %s differs from already installed version (%s), skipping' %
                                                    (to_text(dep_role), dep_role.version, role.name, dep_role.install_info['version']))
                            else:
                                if force_deps:
                                    roles_left.append(dep_role)
                                else:
                                    display.display('- dependency %s is already installed, skipping.' % dep_role.name)
            if not installed:
                display.warning("- %s was NOT installed successfully." % role.name)
                self.exit_without_ignore()
        return 0
    def execute_remove(self):
        """
        removes the list of roles passed as arguments from the local system.
        """
        if not context.CLIARGS['args']:
            raise AnsibleOptionsError('- you must specify at least one role to remove.')
        for role_name in context.CLIARGS['args']:
            role = GalaxyRole(self.galaxy, self.api, role_name)
            try:
                if role.remove():
                    display.display('- successfully removed %s' % role_name)
                else:
                    display.display('- %s is not installed, skipping.' % role_name)
            except Exception as e:
                raise AnsibleError("Failed to remove role %s: %s" % (role_name, to_native(e)))
        return 0
    def execute_list(self):