refactor: remove unnecessary checks (#7875)

Author: HcySunYang

Diff for: src/core/instance/state.js

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