SPIGOT-4411: Document use of null for shulker colours

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftShulker.java

@@ -35,8 +35,6 @@ public class CraftShulker extends CraftGolem implements Shulker {
 
     @Override
     public void setColor(DyeColor color) {
-        Preconditions.checkArgument(color != null, "color");
+        getHandle().getDataWatcher().set(EntityShulker.COLOR, (color == null) ? 16 : color.getWoolData());
-
-        getHandle().getDataWatcher().set(EntityShulker.COLOR, color.getWoolData());
     }
 }