message: Create MapHolder::Empty in MapHolder::with_capacity
  Summary: Empty map should be represented by `MapHolder::Empty`.
diff: @@ -308,7 +308,9 @@ impl<K, V> ExactSizeIterator for MHIntoIter<K, V> {
  impl<K, V> MapHolder<K, V> {
  fn with_capacity(n: usize) -> Self {
  - if n <= THRESHOLD {
  + if n == 0 {
  + MapHolder::Empty
  + } else if n <= THRESHOLD {
  MapHolder::Vec(VecMap::with_capacity(n))
  } else {
  MapHolder::Map(IndexMap::with_capacity_and_hasher(n, Default::default()))
  