Fix possible NPE and clean up more edge interaction checks (Fixes #409)

[Phoenix616]

This fixes a possible NPE in the CollisionUtil (issue #409, occurred for me on latest Paper 1.21.10 too) which was introduced in 61d329e as the lastBlockAABB != null check was moved after accessing it for the first time. (Previously it was checked before)

I also moved more of the edge detection code away from accessing the array indexes directly to the new variables. As those check if the lastBlockBB was null (and technically these checks themselves only check for 0 or 1 so -1 would behave the same as the lastBlockBB null check) I decided to initialize those values to -1 as it's not possible to leave them unassigned but still use them later. Alternatively one could use a ternary checking for null for each individual variable but I felt that is even uglier code.

[Lysandr0]

Thank you so much!
And yeah, that function should get revised later down the road...