fix: fix empty array edge case in normalizeChildren

fix #6790

Author: yyx990803

src/core/vdom/helpers/normalize-children.js

@@ -49,14 +49,16 @@ function normalizeArrayChildren (children: any, nestedIndex?: string): Array<VNo
     lastIndex = res.length - 1
     last = res[lastIndex]
     //  nested
-    if (Array.isArray(c) && c.length > 0) {
-      c = normalizeArrayChildren(c, `${nestedIndex || ''}_${i}`)
-      // merge adjacent text nodes
-      if (isTextNode(c[0]) && isTextNode(last)) {
-        res[lastIndex] = createTextVNode(last.text + (c[0]: any).text)
-        c.shift()
+    if (Array.isArray(c)) {
+      if (c.length > 0) {
+        c = normalizeArrayChildren(c, `${nestedIndex || ''}_${i}`)
+        // merge adjacent text nodes
+        if (isTextNode(c[0]) && isTextNode(last)) {
+          res[lastIndex] = createTextVNode(last.text + (c[0]: any).text)
+          c.shift()
+        }
+        res.push.apply(res, c)
       }
-      res.push.apply(res, c)
     } else if (isPrimitive(c)) {
       if (isTextNode(last)) {
         // merge adjacent text nodes

test/unit/modules/vdom/patch/edge-cases.spec.js

@@ -189,4 +189,13 @@ describe('vdom patch: edge cases', () => {
       expect(vm.$refs.foo.$refs.bar.$el.className).toBe(`hello`)
     }).then(done)
   })
+
+  // #6790
+  it('should not render undefined for empty nested arrays', () => {
+    const vm = new Vue({
+      template: `<div><template v-for="i in emptyArr"></template></div>`,
+      data: { emptyArr: [] }
+    }).$mount()
+    expect(vm.$el.textContent).toBe('')
+  })
 })