fix issue with empty css selectors

[timonbandit]

Selector like that

@media (max-width: 575px) {
  .coupon-filter {
  }
}

break crittr. because of error in CSS module.
reworkcss/css#92

[philipp-winterle]

Thanks.

should have already happend. Wondering why it hits now.

Can you please add a test for this example to have it covered?

[philipp-winterle]

Please add test for empty selector