[BUGFIX] Initialize list of tasks properly

In the SchedulerRepository class, the list of tasks is
initialized as NULL, then used as an array. It must be
initialized as array() instead.

Resolves: #59381
Releases: 2.3
Change-Id: Iad6a398c28a13be664fb9fd292daf64e5196330d
Reviewed-on: https://review.typo3.org/30876
Reviewed-by: Francois Suter
Tested-by: Francois Suter