Fix isset returning false if data collection key had null value

src/Data/DataCollection.php

@@ -71,7 +71,7 @@ public function add(array $keyValuePairs): DataCollectionInterface
      */
     public function has(string $key): bool
     {
-        return isset($this->data[$key]);
+        return array_key_exists($key, $this->data);
     }
 
     /**