Serialise nulls when pretty-printing

Author: costas80

jsonvalidator-common/src/main/java/eu/europa/ec/itb/json/validation/ValidationSpecs.java

@@ -144,7 +144,10 @@ public boolean isProduceAggregateReport() {
     private File prettyPrint(File input) {
         try (FileReader in = new FileReader(input)) {
             JsonElement json = com.google.gson.JsonParser.parseReader(in);
-            Gson gson = new GsonBuilder().setPrettyPrinting().create();
+            Gson gson = new GsonBuilder()
+                    .setPrettyPrinting()
+                    .serializeNulls()
+                    .create();
             String jsonOutput = gson.toJson(json);
             File output = new File(input.getParent(), input.getName() + ".pretty");
             FileUtils.writeStringToFile(output, jsonOutput, StandardCharsets.UTF_8);