Fix grafting not applying stats when trait is rejected (#94188)

## About The Pull Request
Instead of stopping the function when the grafted trait is rejected I
set `new_trait` to `null`
This allows the stat adjustments to always apply
The function still returns the trait or null so the tray can show the
correct message


## Why It's Good For The Game
fix https://github.com/tgstation/tgstation/issues/94060

code/modules/hydroponics/seeds.dm

@@ -570,7 +570,7 @@
 	if(new_trait?.can_add(src))
 		genes += new_trait
 	else
-		return
+		new_trait = null
 
 	// Adjust stats based on graft stats
 	set_lifespan(round(max(lifespan, (lifespan + (2/3)*(snip.plant_dna.lifespan - lifespan)))))