@SuppressWarnings("unchecked")
    @Override
    public void setIfUnset(final SerdeGetter getter) {
        if (keySerializer == null) {
            keySerializer = (Serializer<K>) getter.keySerde().serializer();
        }

        initNullableSerializer(keySerializer, getter);
    }
The serializer that is used for {@link TimestampedKeyAndJoinSide}, which is a combo key format of <timestamp, left/right flag, raw-key>
@param <K> the raw key type
streams/src/main/java/org/apache/kafka/streams/state/internals/TimestampedKeyAndJoinSideSerializer.java