[FOLLOWUP][TASK] Fluidification of Opendocs Toolbar (2nd try)

`list($x, )` does not work on empty arrays, causing `$x` being null then.
The code now checks whether there are actually open docs and then does
the list() stuff.

Resolves: #78051
Releases: master
Change-Id: I73d2c3ca40bea8c3dce2ac1de0ec97e136bf228b
Reviewed-on: https://review.typo3.org/50512


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>