fix: avoid issues with incomplete xml data

src/BusProNet/XmlLoader/TravelXmlLoader.php

@@ -230,6 +230,10 @@ class TravelXmlLoader extends AbstractXmlLoader
 
     public function getRooms(\SimpleXMLElement $xml): array
     {
+        if (false === isset($xml->zimmer)) {
+            return [];
+        }
+
         $rooms = [];
 
         foreach ($xml->zimmer->preis as $item) {