Various small fixes (#8112)

* Make nurse spiders forget target

* Tweak or break AI

* Fixes ice edges

* Allow better mapped-in marker beacons

* Allow mapped-in graffiti

* Add spaceship hull corners

* Improve hull corner color finding slightly

code/modules/persistence/graffiti.dm

@@ -18,7 +18,8 @@
 	..(newloc)
 	if(!isnull(_age))
 		graffiti_age = _age
-	message = _message
+	if(!isnull(_message))
+		message = _message
 	if(!isnull(author))
 		author = _author