Fix missing shulker mapping warning

Fixes #320

core/src/main/java/nl/matsv/viabackwards/api/data/BackwardsMappings.java

@@ -35,7 +35,8 @@ public class BackwardsMappings extends MappingData {
         if (diffMappings != null) {
             JsonObject diffItems = diffMappings.getAsJsonObject("items");
             if (diffItems != null) {
-                backwardsItemMappings = VBMappingDataLoader.loadItemMappings(oldMappings.getAsJsonObject("items"), newMappings.getAsJsonObject("items"), diffItems, true);
+                backwardsItemMappings = VBMappingDataLoader.loadItemMappings(oldMappings.getAsJsonObject("items"),
+                        newMappings.getAsJsonObject("items"), diffItems, shouldWarnOnMissing("items"));
             }
 
             JsonObject diffSounds = diffMappings.getAsJsonObject("sounds");

core/src/main/java/nl/matsv/viabackwards/protocol/protocol1_12_2to1_13/data/BackwardsMappings.java

@@ -105,6 +105,11 @@ public class BackwardsMappings extends nl.matsv.viabackwards.api.data.BackwardsM
         return mappedId;
     }
 
+    @Override
+    protected boolean shouldWarnOnMissing(String key) {
+        return super.shouldWarnOnMissing(key) && !key.equals("items");
+    }
+
     public Int2ObjectMap<String> getStatisticMappings() {
         return statisticMappings;
     }