Remove a unused import and fix docblock for DeferredCallbackCollection (#52808)

Author: seriquynh

src/Illuminate/Foundation/Defer/DeferredCallbackCollection.php

@@ -5,7 +5,6 @@
 use ArrayAccess;
 use Closure;
 use Countable;
-use Illuminate\Support\Collection;
 
 class DeferredCallbackCollection implements ArrayAccess, Countable
 {
@@ -39,7 +38,7 @@ public function invoke(): void
     /**
      * Invoke the deferred callbacks if the given truth test evaluates to true.
      *
-     * @param  \Closure  $when
+     * @param  \Closure|null  $when
      * @return void
      */
     public function invokeWhen(?Closure $when = null): void