[BUGFIX] Prevent PHP Warning for count() in scheduler for sqlite

As serialized_executions might be false instead of an array, check type
first. This ensures there will be no php warning

Resolves: #90220
Releases: master
Change-Id: Ic0bf283cc2ab93b6718b491452d431b5d63114bf
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63234


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>