Fixed: [export] Fix checking of null value: take also empty string value

src/Bundle/ChillActivityBundle/Export/Aggregator/PersonAggregators/ActivityReasonAggregator.php

@@ -134,6 +134,10 @@ class ActivityReasonAggregator implements AggregatorInterface, ExportElementVali
                 return 'reasons' === $data['level'] ? 'Group by reasons' : 'Group by categories of reason';
             }
 
+            if (null === $value || '' === $value) {
+                return '';
+            }
+
             switch ($data['level']) {
                 case 'reasons':
                     $r = $this->activityReasonRepository->find($value);