add unit tests for is_empty method for Account

[tcoratger]

Description

This pull request introduces improvements inspired by the Ethereum execution specification:

• Introduces a Lazy pub static EMPTY_ACCOUNT, initialized on first access, which is now utilized in the is_empty method for more efficient account emptiness checks.
• Unit tests have been added to validate the correctness and functionality of the enhanced is_empty method.

These changes enhance the codebase by optimizing account emptiness checks and providing thorough test coverage.

[Rjected]

I don't think this is correct, because now the bytecode_hash field must be Some for is_empty to return true. this is likely why the is_empty function is not just a comparison with a const value in the first place.

[tcoratger]

I don't think this is correct, because now the bytecode_hash field must be Some for is_empty to return true. this is likely why the is_empty function is not just a comparison with a const value in the first place.

@Rjected My bad, I had forgotten this case. I just fixed it, if you agree I think we can merge this because I put the unit tests to cover these two cases of hash None and KECCAK_EMPTY to no longer cause this confusion.

[Rjected]

I don't think this is correct, because now the bytecode_hash field must be Some for is_empty to return true. this is likely why the is_empty function is not just a comparison with a const value in the first place.

@Rjected My bad, I had forgotten this case. I just fixed it, if you agree I think we can merge this because I put the unit tests to cover these two cases of hash None and KECCAK_EMPTY to no longer cause this confusion.

good idea to add tests, thanks!