[MIRROR] Minor Gravity Fixes [MDB IGNORE] (#20127)

* 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?
/🆑

* Minor Gravity Fixes

---------

Co-authored-by: LemonInTheDark <58055496+LemonInTheDark@users.noreply.github.com>

code/modules/mob/living/living_defines.dm

@@ -224,4 +224,4 @@
 	var/list/imaginary_group = null
 
 	/// What our current gravity state is. Used to avoid duplicate animates and such
-	var/gravity_state = 0
+	var/gravity_state = null