fix: exclude null fields from JSON serialization (#8)

Closes #8.

Author: bartsli

src/main/java/pl/wtx/woocommerce/api/client/config/GsonConfig.java

@@ -12,10 +12,9 @@ public static Gson createGson() {
 
         return new GsonBuilder()
                 .setStrictness(Strictness.LENIENT)
-                .serializeNulls()
                 .registerTypeAdapter(OffsetDateTime.class, new OffsetDateTimeAdapter())
                 .create();
 
     }
 
-}
+}