Empty account must has codehash of `keccak(nil)`

[noel2004]

According to the discussion in this issue, we should make the value codehash of empty account as keccak(nil) rather than 0.

For account gadget, an empty account can be indicated by the EMPTY type tag of mpt gadget row right above it. and we need some additional gates for these constraints: codehash = keccak(nil); balance = 0, nonce = 0, storage = 0

(notice the value of storage in original trie is keccak(rlc([])) while for our zktrie the hash of empty trie is just 0.)