Accomodate for when no users-to-watch-topic are defined for an action

Actions like supress-post will also go through the listener to
see if the topic should be watched. However, when no users need to
be subscribed to a topic after a certain action, the value returned
by $actions->getValue() will be NULL.

onAfterInsertExpectedChange expects that return value to be an array
that it will iterate over, so let's just fix that ;)

Change-Id: I5bd9983f427dfe683b10e055b098212807baaa1b