Fix 'Sourcemap is likely to be incorrect' warnings when using `@tailwindcss/vite` (#20103)

This PR fixes an issue where a bunch of warnings would be shown related
to sourcemaps.

This happens when we are dealing with CSS files that are _not_ Tailwind
CSS roots. In that case, in the `transform` step, we return the `src` of
that module as-is because we didn't modify anything. However, when
nothing changed, you have to return a `NullValue` such as `undefined`.

So this is a stupid little fix, but it should get rid of a bunch of
annoying warnings.

Fixes: #19930

## Test plan

- Added an integration test to mimic the problem
- Other tests still pass
- Tested it against the reproduction provided in #19930


Before:
<img width="1887" height="1763" alt="8oQNG5Lqr2B"
src="https://github.com/user-attachments/assets/2d8af456-4176-4f18-92a3-5327e395ac6b"
/>


After:
<img width="1885" height="1404" alt="8oQND5kWSb4"
src="https://github.com/user-attachments/assets/7dd98ea4-126b-45d4-9411-0afbacd2c797"
/>