Fix race condition in Next.js with --turbopack

[RobinMalfait]

This PR fixes an issue where if you use Next.js with --turbopack a race condition happens because the @tailwindcss/postcss plugin is called twice in rapid succession.

The first call sees an update and does a partial update with the new classes. Next some internal mtimes are updated. The second call therefore doesn't see any changes anymore because the mtimes are the same, therefore it's serving its stale data.

Fixes: #17508

Processing CSS files with only @tailwind utilities;

While working on a unit test for this, we also noticed that the PostCSS client did nothing at all when the input css contained only the @tailwind utilities; directive. It turns out that this was a directive that was not being checked for in the fast-bail code so in this PR we also changed it to account for that.

Test plan

• Tested with the repro provided in Tailwind CSS Not Updating Global Styles on Latest Version #17508
• Added a new unit test that calls into the PostCSS plugin directly for the same change from the same JavaScript run-loop.