Initialize certificates array in Unit class

The array $certificates in the Unit class is now initialized as empty by default. This change is made to avoid any potential issues caused by trying to access or manipulate this array when it's still undefined.

src/Unit.php

@@ -28,7 +28,7 @@ class Unit implements UnitInterface
      *
      * @var array
      */
-    private array $certificates;
+    private array $certificates = [];
 
     /**
      * Contains Statistics object