Revert "refactor: remove unnecessary checks (vuejs#7875)"

This reverts commit 43551b4.

Author: yyx990803

src/core/instance/state.js

@@ -111,8 +111,9 @@ function initProps (vm: Component, propsOptions: Object) {
 
 function initData (vm: Component) {
   let data = vm.$options.data
-  // $options.data is guaranteed to be a function after merge
-  data = vm._data = getData(data, vm)
+  data = vm._data = typeof data === 'function'
+    ? getData(data, vm)
+    : data || {}
   if (!isPlainObject(data)) {
     data = {}
     process.env.NODE_ENV !== 'production' && warn(