Explicit value="" for a language select

The selector in developer.html didn't select '(Auto)' upon restoring
state from the local storage. This later lead to a crash from passing
a null value to `getLanguages`.

tools/developer.html

@@ -32,7 +32,7 @@ <h1>highlight.js developer</h1>
       <textarea>Put code here…</textarea>
       <p>
         <button>Update highlighting</button>
-        Language: <select class="languages"><option>(Auto)</option></select>
+        Language: <select class="languages"><option value="">(Auto)</option></select>
       </p>
     </div>
     <div>