Return block hash on `block_deserialization` success

[moisesPompilio]

Instead of just returning std::optional<bool>, block_deserialization functions could return the block hash. This would allow us to compare hashes for successful deserialization and extend fuzzing to cover hash validation.

Proposed behavior:

• Failed blocks return false or 0.
• Cases that currently return null in std::optional<bool> return an empty string.
• Successful deserialization returns the block hash

[brunoerg]

I think this is a good idea and it's what we were doing on bitcoinfuzz v1 (see https://github.com/brunoerg/bitcoinfuzz/blob/main/btcd_lib/wrapper.go#L21)

[bc1cindy]

I've opened a PR to fix this: #222