only generate __vue_styles__ if has CSS modules

Author: yyx990803

lib/loader.js

@@ -201,12 +201,12 @@ module.exports = function (content) {
   var output = 'var __vue_exports__, __vue_options__\n'
 
   // css modules
-  output += 'var __vue_styles__ = {}\n'
   var cssModules = {}
 
   // add requires for styles
   if (parts.styles.length) {
     output += '\n/* styles */\n'
+    var hasModules = false
     parts.styles.forEach(function (style, i) {
       var moduleName = (style.module === true) ? '$style' : style.module
 
@@ -218,6 +218,10 @@ module.exports = function (content) {
 
       // setCssModule
       if (moduleName) {
+        if (!hasModules) {
+          hasModules = true
+          output += 'var __vue_styles__ = {}\n'
+        }
         if (moduleName in cssModules) {
           loaderContext.emitError('CSS module name "' + moduleName + '" is not unique!')
           output += requireString