(fixes #4939)

Description

the map returned by postcss can be a SourceMapGenerator. Use a function to check and call toJSON() on it.
additionally throw an error if the returned map is not something we can pass on as part of the transform result.

An alternate solution would be to roll back the offending commit as this change would not lead to satisfactory sourcemap support on it's own and getting it done isn't a short term endeavor

Additional context

I wasn't able to come up with a testcase for this due to that fact that this map isn't turning up in any results. vite css sourcemaps are completely broken at the moment, especially for bundled and minified output a lot of things need to be added.

1. capture and merge maps in css bundling
2. pass on sourcemap option to esbuild minifier (careful, esbuild has different options, eg. no "hidden")
3. output css maps on dev somehow, maybe even for css turned to js modules?
4. ... ?

there is an old enhancement ticket about it #2830 maybe it's time to revisit that someday

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.