bdk_esplora: handle empty /blocks response

[notmandatory]

"a rogue Esplora server can make an application crash by returning an empty list of blocks to a /blocks request, as latest_blocks is assumed to never be empty in fetch_block()"

[oleonardolima]

A new error variant was added to rust-esplora-client, that covers this scenario, as per #1725 (comment)

However, I think we could have another issue with refactoring (both fetch_block and point of agreement logic) it posts 1.0 and using a defensive implementation instead of using the .expect, as ValuedMammal previous suggestions.

[notmandatory]

To resolve this issue we need to make a new rust-esplora-client 0.12.0 release and update bdk_esplora to use that dependency right? I propose this can wait until post 1.0.0.

[oleonardolima]

Rob's fix has been included as part of 0.11.0 release, so it's fixed by #1746.