fix deprecated passing null as string type (#204)

Author: trin4ik

src/ArrayToXml.php

@@ -241,7 +241,7 @@ protected function isArrayAllKeySequential(array | string | null $value): bool
  protected function addAttributes(DOMElement $element, array $data): void
  {
  foreach ($data as $attrKey => $attrVal) {
- $element->setAttribute($attrKey, $attrVal);
+ $element->setAttribute($attrKey, $attrVal ?? '');
  }
  }