Prevent empty css

plugins/extract-css.js

@@ -1,4 +1,5 @@
 var fs = require('fs')
+var path = require('path')
 var compiler = require('../lib/compiler')
 
 module.exports = function (b, opts) {
@@ -8,17 +9,23 @@ module.exports = function (b, opts) {
 
   var styles = Object.create(null)
   var outPath = opts.out || opts.o || 'bundle.css'
+  var allowEmpty = opts.allowEmpty || opts.empty || false
 
   b.on('bundle', function (bs) {
     bs.on('end', function () {
-      var css = Object.keys(styles)
-        .map(function (file) { return styles[file] })
-        .join('\n')
-      if (typeof outPath === 'object' && outPath.write) {
-        outPath.write(css)
-        outPath.end()
-      } else if (typeof outPath === 'string') {
-        fs.writeFile(outPath, css, function () {})
+      if (allowEmpty || (!allowEmpty && Object.keys(styles)[0] !== undefined)) {
+        var css = Object.keys(styles)
+          .map(function (file) { return styles[file] })
+          .join('\n')
+        if (typeof outPath === 'object' && outPath.write) {
+          outPath.write(css)
+          outPath.end()
+        } else if (typeof outPath === 'string') {
+          if (!fs.existsSync(path.dirname(outPath))) {
+            fs.mkdirSync(path.dirname(outPath))
+          }
+          fs.writeFile(outPath, css, function () {})
+        }
       }
     })
   })