fix: allow less which is empty or comments only (vuejs#151)

fixes vuejs#132

Author: hmsk

src/style/less/index.js

@@ -19,8 +19,8 @@ export default async function (style, options) {
     )
 
     style.$compiled = {
-        code: css.toString(),
-        map: map.toString()
+        code: css && css.toString() || '',
+        map: map && map.toString() || ''
     }
 
     return style

test/expects/empty-less.css

@@ -0,0 +1,3 @@
+var emptyLess = { template: "<div class=\"goodbye\"></div>",};
+
+export default emptyLess;

test/expects/empty-less.js

@@ -0,0 +1,14 @@
+<template>
+<div class="goodbye"></div>
+</template>
+
+<script>
+export default {}
+</script>
+
+<style lang="less">
+</style>
+
+<style lang="less">
+// Empty essentially
+</style>

test/fixtures/empty-less.vue

@@ -67,7 +67,8 @@ function test(name) {
                 'style',
                 'stylus',
                 'external-script',
-                'postcss'
+                'postcss',
+                'empty-less'
             ].indexOf(name) > -1) {
                 var css = read('expects/' + name + '.css')
                 assert.equal(css.trim(), actualCss.trim(), 'should output style tag content')