grounditems/Lootbeam: handle loading models correctly

models are streamed over js5 so loadModel can return null until it is
downloaded

runelite-api/src/main/java/net/runelite/api/Client.java

@@ -1057,7 +1057,9 @@ public interface Client extends GameEngine
 	 * Loads a model from the cache
 	 *
 	 * @param id the ID of the model
+	 * @return the model or null if it is loading or nonexistent
 	 */
+	@Nullable
 	Model loadModel(int id);
 
 	/**
@@ -1066,7 +1068,9 @@ public interface Client extends GameEngine
 	 * @param id the ID of the model
 	 * @param colorToFind array of hsl color values to find in the model to replace
 	 * @param colorToReplace array of hsl color values to replace in the model
+	 * @return the model or null if it is loading or nonexistent
 	 */
+	@Nullable
 	Model loadModel(int id, short[] colorToFind, short[] colorToReplace);
 
 	/**