Revert "Pass undefined instead of an empty string"

This reverts commit b7a153f88c.

src/compiler/preprocess/index.ts

@@ -95,7 +95,7 @@ export default async function preprocess(
 		source = await replace_async(
 			source,
 			/<!--[^]*?-->|<script(\s[^]*?)?(?:>([^]*?)<\/script>|\/>)/gi,
-			async (match, attributes = '', content) => {
+			async (match, attributes = '', content = '') => {
 				if (!attributes && !content) {
 					return match;
 				}
@@ -115,7 +115,7 @@ export default async function preprocess(
 		source = await replace_async(
 			source,
 			/<!--[^]*?-->|<style(\s[^]*?)?(?:>([^]*?)<\/style>|\/>)/gi,
-			async (match, attributes = '', content) => {
+			async (match, attributes = '', content = '') => {
 				if (!attributes && !content) {
 					return match;
 				}

test/preprocess/samples/script-self-closing/_config.js

@@ -3,7 +3,7 @@ import * as assert from "assert";
 export default {
 	preprocess: {
 		script: ({ content, attributes }) => {
-			assert.equal(content, undefined);
+			assert.equal(content, "");
 			return {
 				code: `console.log("${attributes["the-answer"]}");`
 			};

test/preprocess/samples/style-self-closing/_config.js

@@ -3,7 +3,7 @@ import * as assert from "assert";
 export default {
 	preprocess: {
 		style: ({ content, attributes: { color } }) => {
-			assert.equal(content, undefined);
+			assert.equal(content, "");
 			return {
 				code: `div { color: ${color}; }`
 			};