fix(runtime-vapor): render slot fallback if slot content is not a valid block

close #13668

Author: edison1105

packages/runtime-vapor/src/componentSlots.ts

@@ -126,6 +126,7 @@ export function createSlot(
     const renderSlot = () => {
       const slot = getSlot(rawSlots, isFunction(name) ? name() : name)
       if (slot) {
+        fragment.fallback = fallback
         // create and cache bound version of the slot to make it stable
         // so that we avoid unnecessary updates if it resolves to the same slot
         fragment.update(