[BUGFIX] Properly check serialized_executions to avoid PHP 7.2 warnings

To avoid warnings with PHP 7.2, `serialized_executions` must be checked
for being not an empty value.

Resolves: #83691
Releases: master, 8.7, 7.6
Change-Id: I5855a03d6669ec0e82931cfc0110c2b7ca134ad4
Reviewed-on: https://review.typo3.org/55484


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>