[fix] add filename to combined source map if needed (#6089)

src/compiler/utils/mapped_code.ts

@@ -251,6 +251,11 @@ export function combine_sourcemaps(
 
 	if (!map.file) delete map.file; // skip optional field `file`
 
+	// When source maps are combined and the leading map is empty, sources is not set.
+	// Add the filename to the empty array in this case. 
+	// Further improvements to remapping may help address this as well https://github.com/ampproject/remapping/issues/116
+	if (!map.sources.length) map.sources = [filename];
+
 	return map;
 }
 

test/sourcemaps/samples/preprocessed-no-map/_config.js

@@ -1,5 +1,5 @@
 export default {
-	css_map_sources: [],
+	css_map_sources: ['input.svelte'],
 	preprocess: [
 		{
 			style: ({ content }) => {