Update detection of closure-based scheduled tasks

Use Str to check for empty commands, which will account for the command being null or an empty string.

Also add check for the closure summaryForDisplay output being one of either `Closure` or `Callback`, which was added to support PHP 8

@see laravel/framework#34999

src/TaskIdentifier.php

@@ -19,15 +19,15 @@ class TaskIdentifier
     public function __invoke($task)
     {
         if ($task instanceof CallbackEvent) {
-            if (is_null($task->command) && $task->description && class_exists($task->description)) {
+            if (Str::of($task->command)->isEmpty() && $task->description && class_exists($task->description)) {
                 return static::TYPE_JOB;
             }
 
-            if (is_null($task->command) && Str::is($task->description, $task->getSummaryForDisplay())) {
+            if (Str::of($task->command)->isEmpty() && Str::is($task->description, $task->getSummaryForDisplay())) {
                 return static::TYPE_CLOSURE;
             }
 
-            if (Str::is($task->getSummaryForDisplay(), 'Closure')) {
+            if (in_array($task->getSummaryForDisplay(), ['Closure', 'Callback'])) {
                 return static::TYPE_CLOSURE;
             }
         }
@@ -41,7 +41,7 @@ public function __invoke($task)
         }
     }
 
-    private function sanitisedCommand(string $command): string
+    private function sanitisedCommand(?string $command): string
     {
         return trim(str_replace([
             "'",