fix(Util): remove truthy check before isNaN check

[zaida04]

If an invalid string is passed into resolveColor, it never actually throws a COLOR_CONVERT error and will return NaN. Why is this? Well because of this line:

discord.js/src/util/Util.js

Line 473 in cd393fd

else if (color && isNaN(color)) throw new TypeError('COLOR_CONVERT');

Because NaN isn't a truthy value, we don't get past that first condition to be able to check whether the color is NaN. This PR fixes that.

Status and versioning classification:

• I know how to update typings and have done so, or typings don't need updating