This changes `DenseMapInfo<UUID>::getTombstoneKey()` to return a 1-byte
`{0xFF}` sentinel instead of the empty, default constructed UUID().
Returning the same key for the empty and tombstone value apparently
violates the `DenseMap` invariant.
This changes `DenseMapInfo<UUID>::getTombstoneKey()` to return a 1-byte
`{0xFF}` sentinel instead of the empty, default constructed UUID().
Returning the same key for the empty and tombstone value apparently
violates the `DenseMap` invariant.