persist empty relation as null instead of omitting it from the ES doc…

…ument (#78)

resolve #77

src/Document/DataObjectNormalizerTrait.php

@@ -128,6 +128,8 @@ protected function relationAttributes(Concrete $element, array $fields): array
             $data = is_callable($source) ? $source($element) : $element->get($source);
 
             if ($data === null) {
+                $result[$target] = null;
+
                 continue;
             }