Fix backtrace when the playbook is empty
Fix backtrace when the playbook is empty

While commenting a playbook on gluster.org, I have hit a backtrace
because the yaml document is empty.

    Traceback (most recent call last):
      File "/usr/local/bin/generate_ansible_command.py", line 207, in <module>
        if len(get_hosts_for_role(splitted_path[1], p)) > 0:
      File "/usr/local/bin/generate_ansible_command.py", line 174, in get_hosts_for_role
        host_roles = get_host_roles_dict(playbook_file)
      File "/usr/local/bin/generate_ansible_command.py", line 159, in get_host_roles_dict
        hosts = parse_roles_playbook(playbook_file)
      File "/usr/local/bin/generate_ansible_command.py", line 112, in parse_roles_playbook
        for doc in playbook:
    TypeError: 'NoneType' object is not iterable

mscherer/ansible-role-ansible_bastion,mscherer/ansible-role-ansible_bastion,OSAS/ansible-role-ansible_bastion,OSAS/ansible-role-ansible_bastion