Hooks do not have to return true any more, they can return null

Hooks do not have to return true any more, they can return null

Via the argument given by Krinkle in
https://gerrit.wikimedia.org/r/#/c/274751/1/ImageMap_body.php

Quote: "[Returning true is] obsolete for a while and slowly disappearing
from existing code. Only 'return false' is an explicit signal. The
default is true. This was done because it very often is forgotten and
causes broke in production in catastrophic ways on numerous occasions.
This better reflects the mental model of intent and also makes it more
natural when dealing with hooks such as these, which can't be aborted
and as such don't have a sensible purpose in returning false, which
means returning true can be confusing."

Change-Id: I98308ed9105d904e47db3ac7899412f239c2bf9d