fix for java playground

Author: elisherer

java/playground/src/main/java/co/nlighten/jsontransform/playground/GsonConfiguration.java

@@ -17,6 +17,7 @@ public GsonBuilderCustomizer typeAdapterRegistration() {
         return builder -> {
             builder
                     .setDateFormat(dateTime)
+                    .serializeNulls()
                     .registerTypeAdapter(BigDecimal.class, new BigDecimalTypeAdapter())
                     .setNumberToNumberStrategy(ToNumberPolicy.BIG_DECIMAL)
                     .setObjectToNumberStrategy(ToNumberPolicy.BIG_DECIMAL);

java/playground/src/main/java/co/nlighten/jsontransform/playground/TransformTestRequest.java

@@ -5,7 +5,7 @@
 import java.util.Map;
 
 public class TransformTestRequest {
-    public Object input;
+    public JsonElement input;
     public JsonElement definition;
     public Map<String, Object> additionalContext;
 }