Exit properly with terminal block hash

[terencechain]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

Incorrectly implemented consensus spec that we didn't properly return once terminal block hash conditions are met:

def validate_merge_block(block: BeaconBlock) -> None:
    if TERMINAL_BLOCK_HASH != Hash32():
        # If `TERMINAL_BLOCK_HASH` is used as an override, the activation epoch must be reached.
        assert compute_epoch_at_slot(block.slot) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH
        assert block.body.execution_payload.parent_hash == TERMINAL_BLOCK_HASH
        # WE DID NOT RETURN BELOW
        return 
    pow_block = get_pow_block(block.body.execution_payload.parent_hash)
    assert pow_block is not None
    pow_parent = get_pow_block(pow_block.parent_hash)
    assert pow_parent is not None
    assert is_valid_terminal_pow_block(pow_block, pow_parent)

Which issues(s) does this PR fix?

Fixes #

Other notes for review

Caught by hive test via starting Bellatrix as genesis state. Thanks @marioevz