fix: Ensure identity source map has the same number of fields as the combined source map

[kota65535]

Hi @iFaxity,

In PR #115, we have removed the file field in the identity source map because I thought combined source maps from previous plugins do not have it. However, I found it is not always true.

The file field of a combined source map is added here in combineSourcemaps function. Below is the call tree from this.getCombinedSourcemap.

• TransformContext.getCombinedSourcemap
  • TransformContext#_getCombinedSourcemap
    • combineSourcemaps

The combineSourcemaps function is not called if TransformContext#sourcemapChain is empty or has only 1 element.
So whether the file field exists or not depends on how many previous plugins have output the source maps to be combined, which is practically unpredictable.

The solution is quite simple. If the combined source map has a file field, the identity source map should.
Generally speaking, the two source maps should have the same number of optional fields (file, sourceRoot, and sourceContent) defined in Source Map Revision 3 format.

This PR has the following changes:

• makes an identity source map use the same file and sourceRoot as the combined source map
• ensures sourceContent is sanitized from an identity source map

[iFaxity]

LGTM! Merging!

Thanks again for contributing @kota65535.

[github-actions]

🎉 This PR is included in version 5.0.0-rc.3 🎉

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