fix: should use fallback for scoped slots with single falsy v-if

fix vuejs#9658

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

@@ -60,8 +60,10 @@ function normalizeScopedSlot(normalSlots, key, fn) {
     res = res && typeof res === 'object' && !Array.isArray(res)
       ? [res] // single vnode
       : normalizeChildren(res)
-    return res && res.length === 0
-      ? undefined
+    return res && (
+      res.length === 0 ||
+      (res.length === 1 && res[0].isComment) // #9658
+    ) ? undefined
       : res
   }
   // this is a slot using the new v-slot syntax without scope. although it is

test/unit/features/component/component-scoped-slot.spec.js

@@ -1264,4 +1264,17 @@ describe('Component scoped slot', () => {
       expect(vm.$el.textContent).toMatch(`Content:ok`)
     }).then(done)
   })
+
+  //#9658
+  it('fallback for scoped slot with single v-if', () => {
+    const vm = new Vue({
+      template: `<test v-slot><template v-if="false">hi</template></test>`,
+      components: {
+        Test: {
+          template: `<div><slot>fallback</slot></div>`
+        }
+      }
+    }).$mount()
+    expect(vm.$el.textContent).toMatch('fallback')
+  })
 })