Add @nullable to Item.getChangePayload (#187)

Author: lisawray

library/src/main/java/com/xwray/groupie/Item.java

@@ -187,6 +187,7 @@ public boolean isSameAs(Item other) {
         return getId() == other.getId();
     }
 
+    @Nullable
     public Object getChangePayload(Item newItem) {
         return null;
     }