Skip preupdate/update for PIXI hierarchies in which an ancestor doesn't exist

In a similar vein to the recent "skip updates if the group doesn't exist", this PR does the same thing
but using the PIXI hierarchy.

I use addChild to make sprites with subsprites that move in step. This commit skips updating the subsprites if the parent doesn't exist.

The check with `==false` is to ensure that not returning (aka returning undefined) maintains the old behaviour.