Avoid fatal in ContentHandler::getUndoContent by null content

Revision::getContent can return null, which would here result in a fatal
error. Return false to indicate that this undo is not possible.

Change-Id: I6f6b4b59e86f9762f61c50dd65f81a7256c5255a
Merge "Avoid fatal in ContentHandler::getUndoContent by null content"