/** @return a digest for an empty wave. */
private Digest emptyDigest(WaveViewData wave) {
String title = ModernIdSerialiser.INSTANCE.serialiseWaveId(wave.getWaveId());
String id = ApiIdSerializer.instance().serialiseWaveId(wave.getWaveId());
return new Digest(title, "(empty)", id, Collections.<String> emptyList(), -1L, -1L, 0, 0);
}