fix: ignore invalid URLs (url()) (#663)

Author: kgram

lib/url/escape.js

@@ -1,4 +1,7 @@
 module.exports = function escape(url) {
+    if (typeof url !== 'string') {
+        return url
+    }
     // If url is already wrapped in quotes, remove them
     if (/^['"].*['"]$/.test(url)) {
         url = url.slice(1, -1);

test/helpers.js

@@ -16,7 +16,7 @@ function getEvaluated(output, modules) {
 				return require("../lib/url/escape");
 			if(module.indexOf("-!/path/css-loader!") === 0)
 				module = module.substr(19);
-			if(modules && modules[module])
+			if(modules && module in modules)
 				return modules[module];
 			return "{" + module + "}";
 		});

test/urlTest.js

@@ -125,6 +125,12 @@ describe("url", function() {
 	test("module from url-loader", ".class { background: green url(module) xyz }", [
 		[1, ".class { background: green url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAA) xyz }", ""]
 	], "", { './module': "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAA" });
+	test("module from null-loader (empty object from webpack)", ".class { background: green url(module) xyz }", [
+		[1, ".class { background: green url([object Object]) xyz }", ""]
+	], "", { './module': {} });
+	test("module is null", ".class { background: green url(module) xyz }", [
+		[1, ".class { background: green url(null) xyz }", ""]
+	], "", { './module': null });
 
 	test("background img with url", ".class { background: green url( \"img.png\" ) xyz }", [
 		[1, ".class { background: green url( \"img.png\" ) xyz }", ""]