Synchronously check all transactions to have non-zero length

[etan-status]

As part of newPayload block hash verification, the transactionsRoot is computed by the EL. Because Merkle-Patricia Tries cannot contain [] entries, MPT implementations typically treat setting a key to [] as deleting the entry for the key. This means that if a CL receives a block with transactions containing one or more zero-length transactions, that such transactions will effectively be skipped when computing the transactionsRoot. Note that transactions are opaque to the CL and zero-length transactions are not filtered out before newPayload.

# https://eips.ethereum.org/EIPS/eip-2718
def compute_trie_root_from_indexed_data(data):
    """
    Computes the root hash of `patriciaTrie(rlp(Index) => Data)` for a data array.
    """
    t = HexaryTrie(db={})
    for i, obj in enumerate(data):
        k = encode(i, big_endian_int)
        t.set(k, obj)  # Implicitly skipped if `obj == b''` (invalid RLP)
    return t.root_hash

In any case, the blockHash validation may still succeed, resulting in a potential SYNCING/ACCEPTED result to newPayload by spec.

Note, however, that there is an effective hash collision if a payload is modified by appending one or more zero-length transactions to the end of transactions list: In the trivial case, a block with zero transactions has the same transactionsRoot (and blockHash) as one of a block with one [] transaction (as that one is skipped).

This means that the same blockHash can refer to a valid block (without extra [] transactions added), but also can refer to an invalid block. Because forkchoiceUpdated refers to blocks by blockHash, outcome may be nondeterministic and implementation dependent. If forkchoiceUpdated deems the blockHash to refer to a VALID object (obtained from a src that does not have the extra [] transactions, e.g., devp2p), then this could result in honest attestations to a CL beacon block with invalid [] transactions in its ExecutionPayload, risking finalizing it.

The problem can be avoided by returning INVALID in newPayload if there are any zero-length transactions entries, preventing optimistic import of such blocks by the CL.