[BUGFIX] initializeHookObjects: array not properly checked

When using the scheduler for creating recurring newsletter an error gets logged
when the hook array isn't initialized.

Change-Id: I730d83abde79ff6f16bf39ff89b4f198b04558a5
Resolves: #52094
Releases: 3.1, 4.0
Reviewed-on: https://review.typo3.org/25025
Reviewed-by: Stefan Neufeind
Reviewed-by: Bernhard Kraft
Tested-by: Bernhard Kraft