Merge bitcoindevkit/bdk#1031: remove duplicate `is_empty`

f5074ee3aee319137db1ddfa1e99f7e111abcfd4 remove duplicate `is_empty` from DerivationAdditions (Vladimir Fomene)

Pull request description:

  Duplicate `is_empty` method

  ### 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

ACKs for top commit:
  danielabrozzoni:
    ACK f5074ee3aee319137db1ddfa1e99f7e111abcfd4

Tree-SHA512: 1cb48d25c9e57dbe444646ebc33c838e3d58c2523d74b58a75f0d085f68d070ec6a644c46e0bccb8765af7167c8359f079a6222fc9d17ae28ce1e6dda820fc84