Merge bitcoindevkit/bdk-cli#116: Create wallet dir only when necessary

bff2d375a1dc811b96e0ce47b21ab8ebb17f1d23 `open_database` avoids creating a wallet dir if... ...not necessary (Daniela Brozzoni)

Pull request description:

  ### Description

  Making sure to create the wallet dir only when necessary (database is either sqlite or key-value)

  #### All Submissions:

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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature
  * [ ] I've updated `CHANGELOG.md`

  #### Bugfixes:

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

ACKs for top commit:
  rajarshimaitra:
    tACK bff2d375a1dc811b96e0ce47b21ab8ebb17f1d23

Tree-SHA512: ee019830269f5d8332043ef8de750930883eed1791c867da048bd72c945dc576f74e5178b6c0a27737b0035208f7c862cf44c90321dd2bf6f74900fab08e113b