Brackets on always shouldn't fail if line is empty

[golmansax]

With brackets set to always, my linter was failing:

my-mixin() {
  {block}
}

on the line with {block}. From digging through the source, it seemed like something was stripping out the interpolation, and passing ' ' to the brackets check, where it was failing. I wasn't sure where exactly to address this, but here is my attempt.

[rossPatton]

thanks i'll take a look