Poly speech/memory fix. (#79851)

## About The Pull Request
Simple fix for Poly's AI, Read Memory was returning null into a list
used for Poly's speech causing a stack error. I changed the returned
value to the empty returnable list to negate this problem.

Fixes #79844
Fixes #79859
Fixes #79865

First time making a pull request on Github, please bear with me if i
make any mistakes.

## Why It's Good For The Game
Less errors on the servers side. Probably saving the headache of
debugging from some other poor soul.

---------

Co-authored-by: san7890 <the@san7890.com>

code/modules/mob/living/basic/pets/parrot/poly.dm

@@ -129,7 +129,7 @@
 	else
 		var/json_file = file("data/npc_saves/Poly.json")
 		if(!fexists(json_file))
-			return
+			return list()
 		var/list/json = json_decode(file2text(json_file))
 		returnable_list = json["phrases"]
 		rounds_survived = json["roundssurvived"]