Fix EmptyRequestHash value to match empty set

[MatusKysel]

Description:

This PR corrects the computation of the requests_hash field for block headers. According to EIP-7685, the requests_hash should match the value of an empty sha256() digest if there are no valid requests. Currently, the computed hash does not align with the expected result, and also differs from the implementation in go-ethereum.

Changes:

def compute_requests_hash(block_requests: Sequence[bytes]):
    m = sha256()
    for r in block_requests:
        if len(r) > 1:
            m.update(sha256(r).digest())
    return m.digest()

block.header.requests_hash = compute_requests_hash(requests)