[bot] This addresses and issues that arises when origin.sender is None

This is to address and issue a user reported in IRC:

<kunwon1> i believe i've found a bug in willie, can't handle empty quit messages: http://pastie.org/pastes/8556749/text?key=4higuge7brkfy701kg7kq

Signed-off-by: Eldon McGuinness <eldon.mcguinness@gmail.com>