[FOLLOWUP][TASK] Fluidification of Opendocs Toolbar

The properties `$openDocs` and `$recentDocs` are described and treated
as arrays, but initialized as null values. This patch changes the
declaration to array.

Resolves: #78051
Releases: master
Change-Id: I859c13478b17ddd9fbb7ab7686099f27b3c0e7ea
Reviewed-on: https://review.typo3.org/50511


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>