Leading space for empty format

[favoyang]

Hi @kutuluk,

Thanks for creating this useful lib. I want to give warn and error a prefix.

prefix.apply(log, {
    format(level, name, timestamp) {
      if (level == "error" || level == "warn") {
        return `[${level}]`;
      }
      return "";
    }
  });

result:

[WARN] a warning text
 an info text
[ERROR] an error text

Notice that the info level line has a leading space. how to remove it？