Minor Gravity Fixes (#74285)

## About The Pull Request

Fixes forced gravity not updating a mob (trait was added too early,
before hook signals are registered)
Fixes spawning a mob in space not causing floats (default grav was 0, so
== null was wrong)
Runs shake_everyone() (Our gravity gen reaction hook) AFTER gravity
changes, ensuring mobs hook into it

## Why It's Good For The Game

Closes #74271 
Closes #74272 (Caused by being in nograv but not floating)
Closes #74274 
## Changelog
🆑
fix: Fixes a bunch of minor gravity bugs, report em if you see more
yeah?
/🆑

code/datums/elements/forced_gravity.dm

@@ -13,12 +13,13 @@
 
 	src.gravity = gravity
 	src.ignore_turf_gravity = ignore_turf_gravity
-	ADD_TRAIT(target, TRAIT_FORCED_GRAVITY, REF(src))
 
 	RegisterSignal(target, COMSIG_ATOM_HAS_GRAVITY, PROC_REF(gravity_check))
 	if(isturf(target))
 		RegisterSignal(target, COMSIG_TURF_HAS_GRAVITY, PROC_REF(turf_gravity_check))
 
+	ADD_TRAIT(target, TRAIT_FORCED_GRAVITY, REF(src))
+
 /datum/element/forced_gravity/Detach(datum/source)
 	. = ..()
 	var/static/list/signals_b_gone = list(COMSIG_ATOM_HAS_GRAVITY, COMSIG_TURF_HAS_GRAVITY)