fix: serialize null values

The GSon library do not include by
default the null values, but this
can be need in som plugin like
btcj4cli.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

src/main/java/jrpc/service/converters/JsonConverter.java

@@ -56,7 +56,7 @@ public JsonConverter() {
         CLightningFeeRate.class, new FeeRateTypeAdapter(gsonBuilder.create()));
     gsonBuilder.registerTypeAdapter(
         BitcoinUTXO.class, new BitcoinUTXOTypeAdapter(gsonBuilder.create()));
-    this.gson = gsonBuilder.create();
+    this.gson = gsonBuilder.serializeNulls().create();
   }
 
   @Override