Avoid throwing exception due to deleted change

When a refupdated event is fired due to the deletion of a change (ie.
deleting a draft), the plugin attempts to post a stream event related
to the change, but an unhelpful exception is thrown because the change
no longer exists. Replace exception with a null check.

Change-Id: I7819d3b249c456f9d2749b808a9e67fd4759e3e8