[TASK] Use COUNT(*) in SchedulerTaskRepository

Counting by column name has to check against null
before. To speed up performance a little bit
we switch to COUNT(*).

Resolves: #100297
Releases: main
Change-Id: Ia19e34852bea634de6447c198865a00ed3cb81a2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78253


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Thomas Hohn <tho@gyldendal.dk>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Thomas Hohn <tho@gyldendal.dk>
Tested-by: Nikita Hovratov <nikita.h@live.de>