[BUGFIX] Prevent PHP notice in TreeController

With #81987 we have introduced code, which could cause PHP notices.
This patch changes the check to prevent such notices.

Resolves: #85057
Related: #81987
Releases: master
Change-Id: I85e85501c2593b8abacac1d8296148d03d57f407
Reviewed-on: https://review.typo3.org/57019


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>