Fix mask parent existence check

Attempt to fix parent existence check in masks (all but Pixel/Imagemask & Slopedgrid):
 - added static `_FAKE_PARENT` to Entity
 - changed related code in collision functions to use that if parent == null

This tries to address the fact that masks can be parent-less, but this condition wasn't tested inside collision code (which @MattTuttle started to fix here ed800199fabfcc3029ee7167f999260fb06f2c3a).

What about this approach? 

It is surely more readable/maintanable, but not sure about performance.