fix(runtime-dom): avoid unset option's value

close vuejs#10412
re-fix vuejs#10396

packages/runtime-dom/src/modules/props.ts

@@ -34,13 +34,17 @@ export function patchDOMProp(
     // custom elements may use _value internally
     !tag.includes('-')
   ) {
+    const mounted = '_value' in el
     // store value as _value as well since
     // non-string values will be stringified.
     el._value = value
     // #4956: <option> value will fallback to its text content so we need to
     // compare against its attribute value instead.
-    const oldValue =
-      tag === 'OPTION' ? el.getAttribute('value') || '' : el.value
+    let oldValue = el.value
+    if (tag === 'OPTION') {
+      const attrValue = el.getAttribute('value')
+      oldValue = mounted ? attrValue || '' : attrValue
+    }
     const newValue = value == null ? '' : value
     if (oldValue !== newValue) {
       el.value = newValue