Add placeholder for code_or_symbol to NullColorizer

The signature of `NullColorizer#wrap` was different from `ConsoleCodes#wrap`, therefore `#message_lines` raised error.
I'd prefer if `ignored` was called `_code_or_symbol`.  The `_` is a hint to ruby that it's an ignored variable, and giving it a name related to the `wrap` signature it is aligning with helps make it clearer.