Fixing case where /party chat (on|off) would throw a NullPointerException

If a player types <code>/party chat on</code> or <code>/party chat off</code>, the plugin throws a NullPointerException as the mcMMOPlayer variable isn't set for these cases. This PR fixes that issue by treating it the same way that <code>/p (on|off)</code> is treated.

<code>/party chat [message]</code> is otherwise unaffected.