Merge bitcoindevkit/bdk#757: Enable signing taproot transactions with only `non_witne...

5e9965fca7fb50708af0d293a9f6cd27d0c85fed Enable signing taproot transactions with only `non_witness_utxos` (Alekos Filini)

Pull request description:

  ### Description

  Some wallets may only specify the `non_witness_utxo` for a PSBT input. If that's the case, BDK should still be able to sign.

  This was pointed out in the discussion of #734

  ### Changelog notice

  - Enable signing taproot transactions that only specify the `non_witness_utxo`

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  danielabrozzoni:
    tACK 5e9965fca7fb50708af0d293a9f6cd27d0c85fed - the code looks good to me, I played around with the test you provided (inspecting the PSBT, adding/removing the witness and non-witness utxos, etc) and everything works as expected.

Tree-SHA512: 2f205286263bfee4c76de8e8c81ae1349b1c3b255b72045488f8d629c05cab64c6f775307e831674dc036e5a3a760f95d9cdc1beaf48afb4c475aee838131a33