fix(assistant-v1): override toString method to allow null values

Author: kevinkowa

assistant/src/main/java/com/ibm/watson/assistant/v1/model/UpdateDialogNode.java

@@ -721,4 +721,8 @@ public Date updated() {
   public Map<String, Object> asPatch() {
     return GsonSingleton.getGsonWithSerializeNulls().fromJson(this.toString(), Map.class);
   }
+
+  public String toString() {
+    return GsonSingleton.getGsonWithSerializeNulls().toJson(this);
+  }
 }