Ensure CSS before a layer stays unlayered

[RobinMalfait]

This PR fixes an issue where CSS that existed before a layer:

.foo {
  color: red;
}

@layer components {
  .bar {
    color: blue;
  }
}

Was turned into an @layer without a name:

@layer {
  .foo {
    color: red;
  }
}

@utility bar {
  color: blue;
}

But in this case, it should stay as-is:

.foo {
  color: red;
}

@utility bar {
  color: blue;
}