Handle null damage events for item frames. Fixes BUKKIT-5114

Some types of damage are handled specially so do not end up returning an
event in handleEntityDamageEvent. To ensure we don't have problems with
these we need to check for them and simply ignore them, as they've been
handled already.
Call event when pistons push an item frame/painting. Fixes BUKKIT-5110

When pistons push/pull blocks they call Entity.move(float, float) to move
entities out of their path. For hanging entities this code path makes them
simply die and drop as an item. We now call an event in this scenario so
plugins can control this behavior.