Fix path.join error. Fixes GH-17.

lib/pathcache.js

@@ -98,6 +98,9 @@ function resolvers(options, webpackResolver) {
     function(context, path) {
       // Stylus calls the argument name. If it exists it should match the name
       // of a module in node_modules.
+      if (!path) {
+        return null;
+      }
       var found = utils.lookupIndex(path, options.paths, options.filename);
       if (found) {
         return {path: found, index: true};

test/basic.test.js

@@ -85,12 +85,17 @@ describe("basic", function() {
 	it("should allow stylus plugins to be configured in webpack.config.js", function() {
 		var css = require("!raw-loader!../!./fixtures/webpack.config-plugin.styl");
 		(typeof css).should.be.eql("string");
-                css.should.match(/width:\s?100%;/);
+		css.should.match(/width:\s?100%;/);
 	});
 	it("correctly compiles mixin calls inside imported files", function () {
 		var css = require("!raw-loader!../!./fixtures/import-mixins/index.styl");
 		(typeof css).should.be.eql("string");
 		var regexp = new RegExp('body{color:#639;}.rule{color:#639;}main{color:#639;}');
 		css.replace(/\s/g, '').should.match(regexp);
 	});
+	it("should compile an @import URL through the CSS loader", function () {
+		var css = require("!css-loader!../!./fixtures/import-google-font.styl");
+		(typeof css).should.be.eql("string");
+		css.should.be.eql('@import url(http://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic);\n');
+	});
 });

test/fixtures/import-google-font.styl

@@ -0,0 +1 @@
+@import url(http://fonts.googleapis.com/css?family=Open\+Sans:400,700,400italic)