[BUGFIX] Prevent PHP warnings in initial marker hook

To prevent warnings in the initial marker TCA hook the data property in
the MarkerBase class is casted to an array.

Resolves: #55990
Change-Id: Ib92fed52c5cfd899293d9de6bc6d1a13100ba1c3
Reviewed-on: https://review.typo3.org/27632
Reviewed-by: Alex Kellner
Tested-by: Alex Kellner