Check for closure before class exists (#36293)

* class_exists on a closure throws an exception in PHP8

src/Illuminate/Routing/Router.php

@@ -712,7 +712,13 @@ public function gatherRouteMiddleware(Route $route)
         })->flatten()->reject(function ($name) use ($excluded) {
             if (empty($excluded)) {
                 return false;
-            } elseif (in_array($name, $excluded, true)) {
+            }
+
+            if ($name instanceof Closure) {
+                return false;
+            }
+
+            if (in_array($name, $excluded, true)) {
                 return true;
             }