code: private String getOptionalProperty(JsonObject jsonObject, String key) {
      return jsonObject.has(key) ? jsonObject.get(key).getAsString() : DEFAULT_VALUE_FOR_OPTIONAL_PROPERTY;
    }
docs: Get optional property from a {@link JsonObject} for a {@link String} key.
  If key does'nt exists returns {@link #DEFAULT_VALUE_FOR_OPTIONAL_PROPERTY}.
  @param jsonObject
  @param key
  @return