[!!!][BUGFIX] Correct the return value for getFirst()

This change fixes the return value for getFirst() according to the api to the first object instead of an sliced array.
After this change not an array  but a single node will be returned.

Author: johannessteu

Classes/Flowpack/ElasticSearch/ContentRepositoryAdaptor/Eel/ElasticSearchQueryResult.php

@@ -151,7 +151,7 @@ public function getFirst()
     {
         $this->initialize();
         if (count($this->nodes) > 0) {
-            return array_slice($this->nodes, 0, 1);
+            return array_values($this->nodes)[0];
         }
     }