fix: initial value of template refs in Options API (#2587)

Author: jay-es

src/guide/essentials/template-refs.md

@@ -67,7 +67,7 @@ export default {
 
 </div>
 
-Note that you can only access the ref **after the component is mounted.** If you try to access <span class="options-api">`$refs.input`</span><span class="composition-api">`input`</span> in a template expression, it will be `null` on the first render. This is because the element doesn't exist until after the first render!
+Note that you can only access the ref **after the component is mounted.** If you try to access <span class="options-api">`$refs.input`</span><span class="composition-api">`input`</span> in a template expression, it will be <span class="options-api">`undefined`</span><span class="composition-api">`null`</span> on the first render. This is because the element doesn't exist until after the first render!
 
 <div class="composition-api">