fix(runtime-core): ensure mergeProps skips undefined event handlers (#5299)

fix #5296

packages/runtime-core/__tests__/vnode.spec.ts

@@ -446,6 +446,10 @@ describe('vnode', () => {
         onClick: [clickHandler1, clickHandler2],
         onFocus: focusHandler2
       })
+      let props3: Data = { onClick: undefined }
+      expect(mergeProps(props1, props3)).toMatchObject({
+        onClick: clickHandler1
+      })
     })
 
     test('default', () => {

packages/runtime-core/src/vnode.ts

@@ -798,6 +798,7 @@ export function mergeProps(...args: (Data & VNodeProps)[]) {
         const existing = ret[key]
         const incoming = toMerge[key]
         if (
+          incoming &&
           existing !== incoming &&
           !(isArray(existing) && existing.includes(incoming))
         ) {