[fix] handle preprocess with empty sourcemaps

[bluwy]

Fixes sveltejs/svelte-preprocess#412

Handles preprocessors that return empty sourcemaps like { mappings: '' }, which is valid per rollup documentation.

cc @dominikg

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Tests

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[dominikg]

looks good. is there any way to blame the offending preprocessor and/or notify the user about the potentially broken result?

[dummdidumm]

We thought about adding a warning before but shied away from it because it would probably pollute the output in most cases without the user being able to do much about it

[Conduitry]

This has been released in 3.43.0 - thanks!