Fixed: a bug for range generatoion during week-end

The range generator failed during week-end, because no range is selected (lastStart and endDate is null).

with this commit, the rangeGenerator may return null instead of an array, which means no sms will be send.

src/Bundle/ChillCalendarBundle/Service/ShortMessageNotification/DefaultRangeGenerator.php

@@ -31,14 +31,14 @@ use UnexpectedValueException;
  */
 class DefaultRangeGenerator implements RangeGeneratorInterface
 {
-    public function generateRange(\DateTimeImmutable $date): array
+    public function generateRange(\DateTimeImmutable $date): ?array
     {
         $onMidnight = DateTimeImmutable::createFromFormat('Y-m-d H:i:s', $date->format('Y-m-d') . ' 00:00:00');
 
         switch ($dow = (int) $onMidnight->format('w')) {
             case 6: // Saturday
             case 0: // Sunday
-                return ['startDate' => null, 'endDate' => null];
+                return null;
 
             case 1: // Monday
                 // send for Tuesday and Wednesday