fix: handle null module.layer in renderContentAsset (#1159)

Author: Vyazovoy

src/index.js

@@ -1537,11 +1537,11 @@ class MiniCssExtractPlugin {
         if (
           module.media ||
           module.supports ||
-          typeof module.layer !== "undefined"
+          typeof module.layer === "string"
         ) {
           let atImportExtra = "";
 
-          const needLayer = typeof module.layer !== "undefined";
+          const needLayer = typeof module.layer === "string";
 
           if (needLayer) {
             atImportExtra +=
@@ -1577,7 +1577,7 @@ class MiniCssExtractPlugin {
           source.add(`@media ${module.media} {\n`);
         }
 
-        const needLayer = typeof module.layer !== "undefined";
+        const needLayer = typeof module.layer === "string";
 
         if (needLayer) {
           source.add(

test/cases/simple-with-experiments-layers/expected/main.css

@@ -0,0 +1,4 @@
+body {
+  background: red;
+}
+

test/cases/simple-with-experiments-layers/index.js

@@ -0,0 +1 @@
+import "./style.css";

test/cases/simple-with-experiments-layers/style.css

@@ -0,0 +1,3 @@
+body {
+  background: red;
+}

test/cases/simple-with-experiments-layers/webpack.config.js

@@ -0,0 +1,21 @@
+import Self from "../../../src";
+
+module.exports = {
+  entry: "./index.js",
+  experiments: {
+    layers: true,
+  },
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [Self.loader, "css-loader"],
+      },
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: "[name].css",
+    }),
+  ],
+};