Merge bitcoindevkit/bdk#571: Fix pre-segwit inputs with esplora

52bc63e48f5d16cb03c8a58f7b0bd07cbb058dde Fix pre-segwit inputs with esplora (LLFourn)

Pull request description:

  Unexpectedly pre-segwit inputs have an empty JSON witness field in esplora.

  Fixes #570

  * [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:
  notmandatory:
    ACK 52bc63e48f5d16cb03c8a58f7b0bd07cbb058dde
  afilini:
    ACK 52bc63e48f5d16cb03c8a58f7b0bd07cbb058dde

Tree-SHA512: 1da82aeb2739111e1a516d94c79fe7d7c7879526a8cd780dcd63ff5ae9ccb1bed4eb20e5c575a45e37b6d1818d63ce6d3812e7b9ae34ebb02bc190a47b9750f8