pub fn is_special_outpoint(outpoint: OutPoint) -> bool {
    outpoint == OutPoint::null() || outpoint == unbound_outpoint()
  }
Unlike normal outpoints, which are added to index on creation and removed
when spent, the UTXO entry for special outpoints may be updated.

The special outpoints are the null outpoint, which receives lost sats,
and the unbound outpoint, which receives unbound inscriptions.
src/index.rs