fix slug

src/Entity/Entity.php

@@ -4,9 +4,13 @@ namespace Bluesquare\Pilot\Entity;
 
 abstract class Entity
 {
-    public function __construct(protected $slug = null)
-    {
+    protected $slug = null;
 
+    public function __construct($slug = null)
+    {
+        if (! is_null($slug)) {
+            $this->slug = $slug;
+        }
     }
 
     public function slug()