String getArg(String key, String defaultValue) {
    String value = getArg(key);
    return value == null ? defaultValue : value;
  }
Chain two argument providers so that {@code other} is used as a fallback to {@code this}.

@param other another arguments provider
@return arguments instance that checks {@code this} first and if a match is not found then {@code other}
planetiler-core/src/main/java/com/onthegomap/planetiler/config/Arguments.java