Merge bitcoindevkit/bdk#1961: Add `is_empty` methods to `TxUpdate` and `{}_Response...

e9263f9b70271cbcfd67c2f5b250c447ba1fd4fe feat(core): Add `is_empty` methods to `TxUpdate` and `{}_Response` types (志宇)

Pull request description:

  ### Description

  `is_empty` methods are helpful in various contexts so I added them.

  ### Changelog notice

  ```md
  Added
    - `is_empty` methods to `TxUpdate`, `SyncResponse` and `FullScanResponse`
  ```

  ### 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

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  LagginTimes:
    ACK e9263f9b70271cbcfd67c2f5b250c447ba1fd4fe

Tree-SHA512: 767a581b5d66da31f609db3ffba2457cb0f579c386e8e960bd89d6650852ba9d3c00e94432aaeba586d19b01fa8982ffdee2615c867b98ad8efa8e16505e3180

diff --cc crates/core/src/spk_client.rs
Simple merge

diff --cc crates/core/src/tx_update.rs
Simple merge