Fixes runtime when trying to pull a turf (#96420)

## About The Pull Request

No type check prior

## Why It's Good For The Game

Runtime

## Changelog
🆑

fix: fixed a runtime when trying to pull a turf

/🆑

code/modules/mob/living/living.dm

@@ -374,8 +374,9 @@
 	now_pushing = FALSE
 
 /mob/living/start_pulling(atom/movable/AM, state, force = pull_force, supress_message = FALSE)
-	if(!AM || !src)
+	if(!src)
 		return FALSE
+	ASSERT(ismovable(AM), "[src] attempted to pull [AM ? "[AM], a nonmovable atom" : "a null object"]")
 	if(!(AM.can_be_pulled(src, force)))
 		return FALSE
 	if(throwing || !(mobility_flags & MOBILITY_PULL))