Nullblades and proto-kinetic crushers use the check_behind() proc rather than get direction themselves. (#87287)

## About The Pull Request

What it says in the title.

## Why It's Good For The Game

This is functionally identical, but a little more robust about what
counts as 'behind'.

## Changelog
🆑
code: Nullblades and proto-kinetic crushers use the new proc for
checking valid directions to backstab.
/🆑

code/modules/mining/equipment/kinetic_crusher.dm

@@ -122,10 +122,9 @@
 	new /obj/effect/temp_visual/kinetic_blast(get_turf(target))
 	var/backstabbed = FALSE
 	var/combined_damage = detonation_damage
-	var/backstab_dir = get_dir(user, target)
 	var/def_check = target.getarmor(type = BOMB)
 	// Backstab bonus
-	if((user.dir & backstab_dir) && (target.dir & backstab_dir) || boosted_mark)
+	if(check_behind(user, target) || boosted_mark)
 		backstabbed = TRUE
 		combined_damage += backstab_bonus
 		playsound(user, 'sound/items/weapons/kinetic_accel.ogg', 100, TRUE) //Seriously who spelled it wrong