Make limit parameter optional

Unification function parameters with a similar method from the repository.
[MAILPOET-4367]

mailpoet/lib/Newsletter/Sending/ScheduledTasksRepository.php

@@ -239,7 +239,7 @@ class ScheduledTasksRepository extends Repository {
   /**
    * @return ScheduledTaskEntity[]
    */
-  public function findScheduledSendingTasks(int $limit): array {
+  public function findScheduledSendingTasks(?int $limit = null): array {
     $now = Carbon::createFromTimestamp($this->wp->currentTime('timestamp'));
     return $this->doctrineRepository->createQueryBuilder('st')
       ->select('st')