fix: include empty string in regexp

Author: jkzing

lib/codegen/styleInjection.js

@@ -1,6 +1,6 @@
 const { attrsToQuery } = require('./utils')
 const hotReloadAPIPath = JSON.stringify(require.resolve('vue-hot-reload-api'))
-const allWhitespaceRE = /^\s+$/
+const allWhitespaceRE = /^\s*?$/
 
 module.exports = function genStyleInjectionCode (
   loaderContext,