fix(hydration): should not warn mismatch for nullish prop

packages/runtime-core/__tests__/hydration.spec.ts

@@ -1397,6 +1397,7 @@ describe('SSR hydration', () => {
       mountWithHydration(`<div spellcheck></div>`, () =>
         h('div', { spellcheck: '' })
       )
+      mountWithHydration(`<div></div>`, () => h('div', { id: undefined }))
       // boolean
       mountWithHydration(`<select multiple></div>`, () =>
         h('select', { multiple: true })

packages/runtime-core/src/hydration.ts

@@ -740,7 +740,9 @@ function propHasMismatch(el: Element, key: string, clientValue: any): boolean {
       ? includeBooleanAttr(clientValue)
         ? ''
         : false
-      : String(clientValue)
+      : clientValue == null
+        ? false
+        : String(clientValue)
     if (actual !== expected) {
       mismatchType = `attribute`
       mismatchKey = key