fix: handle async placeholders in normalizeScopedSlot (#11963)

Co-authored-by: Eduardo San Martin Morote <posva@users.noreply.github.com>

src/core/vdom/helpers/normalize-scoped-slots.js

@@ -3,6 +3,7 @@
 import { def } from 'core/util/lang'
 import { normalizeChildren } from 'core/vdom/helpers/normalize-children'
 import { emptyObject } from 'shared/util'
+import { isAsyncPlaceholder } from './is-async-placeholder'
 
 export function normalizeScopedSlots (
   slots: { [key: string]: Function } | void,
@@ -60,9 +61,10 @@ function normalizeScopedSlot(normalSlots, key, fn) {
     res = res && typeof res === 'object' && !Array.isArray(res)
       ? [res] // single vnode
       : normalizeChildren(res)
+    let vnode: VNode = res && res[0]
     return res && (
-      res.length === 0 ||
+      !vnode ||
-      (res.length === 1 && res[0].isComment) // #9658
+      (vnode.isComment && !isAsyncPlaceholder(vnode)) // #9658, #10391
     ) ? undefined
       : res
   }