Modularize NullColorizer

Other colorizers s.t. ConsoleCodes are module with module_functions.
NullColorizer was a class and the class does not respond to `wrap()`. This
patch fixes the mismatch.

I approached a problem about NullColorizer in PR #1637, but the fix was not complete.
I am sorry about my carelessness.
Yes but it's not clear why you changed this given there wasn't a failing spec before.