This PR is an optimization where it will not emit empty rules and
at-rules. I noticed this while working on
#16120 where we emitted:
```css
:root, :host {
}
```

There are some exceptions for "empty" at-rules, such as:

```css
@charset "UTF-8";
@layer foo, bar, baz;
@Custom-Media --modern (color), (hover);
@namespace "http://www.w3.org/1999/xhtml";
```

These don't have a body, but they still have a purpose and therefore
they will be emitted.

However, if you look at this:

```css
/* Empty rule */
.foo {
}

/* Empty rule, with nesting */
.foo {
  .bar {
  }
  .baz {
  }
}

/* Empty rule, with special case '&' rules */
.foo {
  & {
    &:hover {
    }
    &:focus {
    }
  }
}

/* Empty at-rule */
@media (min-width: 768px) {
}

/* Empty at-rule with nesting*/
@media (min-width: 768px) {
  .foo {
  }

  @media (min-width: 1024px) {
    .bar {
    }
  }
}
```

None of these will be emitted.

---------

Co-authored-by: Jordan Pittman <jordan@cryptica.me>