add validation to aggregator and filters

Export/Filter/GenderFilter.php

@@ -49,11 +49,19 @@ class GenderFilter implements FilterInterface
                 Person::FEMALE_GENDER => 'Woman',
                 Person::MALE_GENDER   => 'Man'
             ),
-            'multiple' => true,
+            'multiple' => false,
             'expanded' => false
         ));
     }
     
+    public function validateForm($data, ExecutionContextInterface $context)
+    {
+        if (count($data['accepted_genders']) === 0) {
+            $context->buildViolation("You should select an option")
+                ->addViolation();
+        }
+    }
+    
     public function alterQuery(QueryBuilder $qb, $data)
     {
         $where = $qb->getDQLPart('where');