fix error in validation

Export/Filter/GenderFilter.php

@@ -61,7 +61,7 @@ class GenderFilter implements FilterInterface,
 
     public function validateForm($data, ExecutionContextInterface $context)
     {
-        if (count($data['accepted_genders']) === 0) {
+        if ($data['accepted_genders'] === null) {
             $context->buildViolation("You should select an option")
                 ->addViolation();
         }