Fix when json/jackson bean inserted as null and not changed

Expectation is that it is not included in update (still null, no change)

ebean-core/src/main/java/io/ebeaninternal/server/deploy/BeanPropertyJsonMapper.java

@@ -57,6 +57,9 @@ public class BeanPropertyJsonMapper extends BeanProperty {
     final String json = scalarType.format(value);
     final MutableValueInfo oldHash = ebi.mutableInfo(propertyIndex);
     if (oldHash == null) {
+      if (value == null) {
+        return false; // no change, still null
+      }
       ebi.mutableNext(propertyIndex, next(json));
       return true;
     }