go.dev: fix uncaught JS error

The searchBox.js code throws an error when no
search form is present on the page. Return early
when the form is not present to prevent the error.

Change-Id: I8a77d34fad185d5c59ab10bce88776d363bb3bef
Reviewed-on: https://go-review.googlesource.com/c/website/+/359215
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>

go.dev/_content/js/searchBox.js

@@ -3,9 +3,11 @@
   const BREAKPOINT = 512;
   const logo = document.querySelector('.js-headerLogo');
   const form = document.querySelector('.js-searchForm');
+  if (!form) return;
   const button = document.querySelector('.js-searchFormSubmit');
   const input = form.querySelector('input');
 
+
   renderForm();
 
   window.addEventListener('resize', renderForm);