[export] add a grouping of accompanying period by opening and closing date

src/Bundle/ChillActivityBundle/Export/Aggregator/DateAggregator.php

@@ -73,7 +73,6 @@ class DateAggregator implements AggregatorInterface
             'choices' => self::CHOICES,
             'multiple' => false,
             'expanded' => true,
-            'empty_data' => self::DEFAULT_CHOICE,
         ]);
     }