stylefmt ignores NULL in rules?

[KayLeung]

stylefmt ignores NULL config and always append an empty line after CSS rule.

{
 "rule-empty-line-before": null
}

the formatting function doesn't check on NULL config? https://github.com/morishitter/stylefmt/blob/master/lib/formatRules.js#L11

Testing Code: (we expected no changes in below code)

/**
 * @file
 * The .contextual.css file is intended to contain styles that override declarations
 * in the Contextual module.
 */

.views-live-preview .contextual-region-active {
  outline: medium none;
}
.views-live-preview .contextual {
  right: auto; /* LTR */
  top: auto;
}