Make extraData parameter optional in Block Format

[saiakhil2012]

extraData parameter inside block is expected as mandatory

An interaction with a Blockchain such as writing new information into Blockchain using a Signer, fails at Block format check if extraData parameter is not present in the Block returned (i.e when eth_getBlockByNumber is called)

extraData parameter should be treated as optional

Also, just a note, in other libraries such as Web3.js or Web3j, the extraData parameter is not treated as mandatory

This is to address the issue - https://github.com/ethers-io/ethers.js/issues/3265