* #lengthIfKnown}, returning true if it's known to be zero and false if it's known to be
   * non-zero. If the length is not known, it falls back to opening a stream and checking for EOF.
   *
   * <p>Note that, in cases where {@code lengthIfKnown} returns zero, it is <i>possible</i> that
   * chars are actually available for reading. This means that a source may return {@code true} from