test(reactivity): test for ref without init value (#746)

packages/reactivity/__tests__/ref.spec.ts

@@ -33,6 +33,17 @@ describe('reactivity/ref', () => {
     expect(dummy).toBe(2)
   })
 
+  it('should work without initial value', () => {
+    const a = ref()
+    let dummy
+    effect(() => {
+      dummy = a.value
+    })
+    expect(dummy).toBe(undefined)
+    a.value = 2
+    expect(dummy).toBe(2)
+  })
+
   it('should work like a normal property when nested in a reactive object', () => {
     const a = ref(1)
     const obj = reactive({