Support a mode being removed from a channel, without an arg, that we weren't yet

aware of (src/IRCChannel.py)

src/IRCChannel.py

@@ -64,7 +64,7 @@ class Channel(IRCObject.Object):
             else:
                 self.modes[mode].add(arg.lower())
     def remove_mode(self, mode: str, arg: str=None):
-        if not arg:
+        if not arg and mode in self.modes:
             del self.modes[mode]
         else:
             if mode in self.server.prefix_modes: