[zk-sdk] Allow all-zero ciphertext for ciphertext-ciphertext equality proof

[samkim-crypto]

Problem

Terribly sorry, this is a quick follow-up to solana-program/token-2022#915. I went through all the token-2022 instructions to double check that we don't use all-zero (identity points) public keys or ciphertexts in any of the token-2022 application. But since solana-program/token-2022#913 was not merged, I missed that we would need to allow all-zero ciphertexts for the ciphertext-ciphertext validity proof.

Summary of Changes

• Updated the ciphertext-ciphertext validity proof to allow all-zero ciphertexts
• While I was at it, I just made the ciphertext identity check more complete for equality proof checks: before, we were just checking if the commitment component was zero, now we check the decryption handle component as well.