fix(custom-element): avoid setting attr to null if it is removed (#9012)

Partially fixes #9006
Fixes #10324

packages/runtime-dom/__tests__/customElement.spec.ts

@@ -139,6 +139,12 @@ describe('defineCustomElement', () => {
       expect(e.shadowRoot!.innerHTML).toBe('<div></div><div>two</div>')
       expect(e.hasAttribute('foo')).toBe(false)
 
+      e.foo = undefined
+      await nextTick()
+      expect(e.shadowRoot!.innerHTML).toBe('<div></div><div>two</div>')
+      expect(e.hasAttribute('foo')).toBe(false)
+      expect(e.foo).toBe(undefined)
+
       e.bazQux = 'four'
       await nextTick()
       expect(e.shadowRoot!.innerHTML).toBe('<div></div><div>four</div>')

packages/runtime-dom/src/apiCustomElement.ts

@@ -313,7 +313,7 @@ export class VueElement extends BaseClass {
   }
 
   protected _setAttr(key: string) {
-    let value = this.getAttribute(key)
+    let value = this.hasAttribute(key) ? this.getAttribute(key) : undefined
     const camelKey = camelize(key)
     if (this._numberProps && this._numberProps[camelKey]) {
       value = toNumber(value)