[fix] handle preprocess with empty sourcemaps (#6757)

src/compiler/preprocess/index.ts

@@ -79,12 +79,15 @@ function processed_content_to_code(processed: Processed, location: SourceLocatio
 	if (processed.map) {
 		decoded_map = decode_map(processed);
 
+		// decoded map may not have sources for empty maps like `{ mappings: '' }`
+		if (decoded_map.sources) {
 			// offset only segments pointing at original component source
 			const source_index = decoded_map.sources.indexOf(file_basename);
 			if (source_index !== -1) {
 				sourcemap_add_offset(decoded_map, location, source_index);
 			}
 		}
+	}
 
 	return MappedCode.from_processed(processed.code, decoded_map);
 }

test/preprocess/samples/empty-sourcemap/_config.js

@@ -0,0 +1,7 @@
+export default {
+	preprocess: {
+		style: ({ content }) => {
+			return { code: content, map: { mappings: '' } };
+		}
+	}
+};

test/preprocess/samples/empty-sourcemap/input.svelte

@@ -0,0 +1,7 @@
+<div class="foo">bar</div>
+
+<style>
+	.foo {
+		color: red;
+	}
+</style>

test/preprocess/samples/empty-sourcemap/output.svelte

@@ -0,0 +1,7 @@
+<div class="foo">bar</div>
+
+<style>
+	.foo {
+		color: red;
+	}
+</style>