Fix PhabricatorBot ignore messages from senders

Summary:
PhabricatorBotMessage->getSender returns a PhabricatorBotUser object (which potentially can be null)
So check null and then use getName to get actual name of the sender

Test Plan: Run phabot and add myself to ignore list

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5036