fix bug assuming merge thing always and only operates on json objects

Author: thjaeckle

things/service/src/main/java/org/eclipse/ditto/things/service/persistence/actors/strategies/commands/MergeThingStrategy.java

@@ -148,7 +148,8 @@ private Result<ThingEvent<?>> applyMergeCommand(final Context<ThingId> context,
 
         final JsonValue finalMergeValue = applyEmptyObjectRemovalIfConfigured(mergeValue);
 
-        if (removeEmptyObjectsAfterPatchConditionFiltering && finalMergeValue.asObject().isEmpty()) {
+        if (removeEmptyObjectsAfterPatchConditionFiltering &&
+                finalMergeValue.isObject() && finalMergeValue.asObject().isEmpty()) {
             final CompletionStage<WithDittoHeaders> responseStage = CompletableFuture.completedFuture(
                     appendETagHeaderIfProvided(command, MergeThingResponse.of(command.getEntityId(), path,
                             createCommandResponseDittoHeaders(dittoHeaders, nextRevision)), thing)