Fix JS error when searchbox is not present

When the search box is not present (e.g.,
https://docs.racket-lang.org/demo-m1/index.html),
pressing "S" will result in a JS error. This PR fixes the problem.
Note that semantically it makes more sense to give an ID to the
search box as we know exactly what search box we want.

scribble-lib/scribble/html-render.rkt

@@ -225,6 +225,7 @@
     `(form ([class "searchform"])
        (input
         ([class "searchbox"]
+         [id "searchbox"]
          [type "text"]
          [tabindex "1"]
          [placeholder ,emptylabel]

scribble-lib/scribble/scribble-common.js

@@ -173,8 +173,10 @@ AddOnLoad(function(){
 AddOnLoad(function(){
   window.addEventListener("keyup", function(e) {
     if ((e.key === 's' || e.key === 'S') && e.target === document.body) {
-      var field = document.getElementsByClassName("searchbox")[0];
-      field.focus();
+      var searchBox = document.getElementById('searchbox');
+      if (searchBox) {
+        searchBox.focus();
+      }
     }
   }, false);
 });