When supports-color detects a level of 0, it will not return an object containing the key `level`, but instead only `false`. This commit makes sure `supportsColor.level` exists before calling `supportsColor.level.toString()`.

The TypeScript definitions of `chalk` seem wrong in this regard, since they imply that if `supportsColor` is truthy, it is an object and always contains the `level` key of type `0 | 1 | 2 | 3`.