groupname is NoneType when not set

When groupname: is not specified, as in the following task...

```
    - name: Add this thing to inventory
      local_action:
        module: add_host
        hostname: "{{ item.name }}"
        ansible_ssh_host: "{{ item.accessIPv4 }}"
        # bug!
        groupname:
      with_items: rax.success
      when: rax.action == 'create'
```

... the following traceback occurs.

```
TASK: [Add this thing to inventory] ******************************************* 
fatal: [localhost] => Traceback (most recent call last):
  File "/home/rstrox/.venv27/lib/python2.7/site-packages/ansible/runner/__init__.py", line 543, in _executor
    exec_rc = self._executor_internal(host, new_stdin)
  File "/home/rstrox/.venv27/lib/python2.7/site-packages/ansible/runner/__init__.py", line 675, in _executor_internal
    complex_args=complex_args
  File "/home/rstrox/.venv27/lib/python2.7/site-packages/ansible/runner/__init__.py", line 839, in _executor_internal_inner
    result = handler.run(conn, tmp, module_name, module_args, inject, complex_args)
  File "/home/rstrox/.venv27/lib/python2.7/site-packages/ansible/runner/action_plugins/add_host.py", line 81, in run
    for group_name in groupnames.split(","):
AttributeError: 'NoneType' object has no attribute 'split'


FATAL: all hosts have already failed -- aborting
```