adjust ignoredElements check after merge

Author: yyx990803

src/core/vdom/patch.js

@@ -102,7 +102,7 @@ export function createPatchFunction (backend) {
         if (
           !inPre &&
           !vnode.ns &&
-          !(config.ignoredElements && config.ignoredElements.indexOf(tag) > -1) &&
+          !(config.ignoredElements.length && config.ignoredElements.indexOf(tag) > -1) &&
           config.isUnknownElement(tag)
         ) {
           warn(