fix: allows to set a custom attribute to null

src/Object/UserAttributes.php

@@ -94,7 +94,7 @@ public function jsonSerialize(): mixed
      * Using a DateTime object it would not be possible to know if you want
      * to send a date that includes time or not.
      */
-    public function setCustomAttribute(string $key, array|bool|float|int|string $value): void
+    public function setCustomAttribute(string $key, array|bool|float|int|string|null $value): void
     {
         $this->_customAttributes[$key] = $value;
     }