Better preservation/fallback of/for source maps

Is your feature request related to a problem? Please describe. Some preprocessors might not return a source map. This breaks the "chain" of source maps and the result is that positions due to errors for example cannot be mapped correctly anymore.

Describe the solution you'd like Investigate if there's a possibility to provide somewhat sane fallbacks for Style and Script preprocessors, like "return a source map that indicates that script/style now has this length instead of the previous one". Don't know how to phrase this better, just an idea.

How important is this feature to you? Nice to have