Problem: empty topics shouldn't be encoded as nil

[yihuang]

System info: ethermint main

Steps to reproduce:

    function test_log0() public {
        bytes memory data = "hello world";
        assembly {
            log0(data, 11)
        }
    }

• use log0 to emit a log without topics
• use web3.py to fetch tx receipt
• it complains: "Could not format invalid type of None for field 'topics'", because rpc API return topics as None, rather than [].

Expected behavior: empty topics return as []

Actual behavior: empty topics return as null

Additional info: