Don’t crash when important and parent selectors are equal in @apply

[thecrypticace]

Previously, when you:

1. Have configured an important selector; AND
2. Add a component to the components layer; AND
3. The component's selector is the same as the important selector; AND
4. It uses @apply

We would crash because we'd end up replacing a selector in a way that it became empty. This PR fixes this problem, we no longer crash, and now generate the expected CSS.

Before — with important: ':root' this would crash:

@tailwind components;
@layer components {
  :root {
    @apply flex;
  }
}

Now it generates the following:

:root {
  display: flex;
}

Fixes #12110