code: @Override
    public ImmutableMap<C, V> row(R rowKey) {
      checkNotNull(rowKey);
      return MoreObjects.firstNonNull(
          (ImmutableMap<C, V>) rowMap().get(rowKey), ImmutableMap.<C, V>of());
    }
docs: {@inheritDoc}
  
  @throws NullPointerException if {@code rowKey} is {@code null}