[BUGFIX] getTreeList must return comma-separated list

While retrieving the list with begin as argument for a
recursive call, getTreeList builds the comma-separated list
correctly.

Resolves: #83328
Releases: master, 8.7, 7.6
Change-Id: I643477dd5c0cbfb67532dd96eb0b479e0cb532b8
Reviewed-on: https://review.typo3.org/56139


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Alexander Schnitzler <typo3@alexanderschnitzler.de>
Tested-by: Alexander Schnitzler <typo3@alexanderschnitzler.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>