code: @Nullable
      @Size(2)
      static String optCountryCode(
              @NonNull JSONObject jsonObject,
              @NonNull @Size(min = 1) String fieldName) {
          String value = nullIfNullOrEmpty(jsonObject.optString(fieldName));
          if (value != null && value.length() == 2) {
              return value;
          }
          return null;
      }
docs: Calls through to {@link JSONObject#optString(String)} while safely converting
  the raw string "null" and the empty string to {@code null}, along with any value that isn't
  a two-character string.
  @param jsonObject the object from which to retrieve the country code
  @param fieldName the name of the field in which the country code is stored
  @return a two-letter country code if one is found, or {@code null}