fix: correct eth_call serialising + contract test

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• fix the eth_call method, partially to adhere to https://www.quicknode.com/docs/ethereum/eth_call and partially to adhere what we have in Filecoin.
• added smart contract testing with case where to is set to None.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[elmattic]

What is the rationale for supporting the Ethereum API? Do we want to extend this to other eth_xxx methods as well?

[LesnyRumcajs]

What is the rationale for supporting the Ethereum API? Do we want to extend this to other eth_xxx methods as well?

In general, the entire eth_ is there to support the Ethereum API. We want to support it as closely as possible, which usually means supporting what Lotus supports (but potentially having better support if it doesn't break compatibility between Forest and Lotus).