Refactor getInstance method to use null coalescing assignment (#52693)

Co-authored-by: Xurshudyan <v.khurshudyan@aico.swiss>

src/Illuminate/Container/Container.php

@@ -1519,11 +1519,7 @@ public function flush()
      */
     public static function getInstance()
     {
-        if (is_null(static::$instance)) {
-            static::$instance = new static;
-        }
-
-        return static::$instance;
+        return static::$instance ??= new static;
     }
 
     /**