Forbid search with empty criteria.

templates/default/header.html

@@ -22,7 +22,7 @@
                     {{? it.databaseId != ""}}
                         <input type="hidden" name="db" value="{{=it.databaseId}}" />
                     {{?}}
-                    <input type="search" name="query" />
+                    <input type="search" required="required" name="query" />
                 </div>
             </form>
         </div>