[BUGFIX] Correctly check for a key in getPageForRootline

Move coalescing operator to inside is_array. It was was introduced
in https://review.typo3.org/#/c/54558/

Releases: master, 8.7
Resolves: #84868
Related: #82921
Change-Id: I54e60cd3fbd2ab36e945cd2e2ed9c6fc02a5bf5a
Reviewed-on: https://review.typo3.org/56804


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Riny van Tiggelen <info@online-gamer.nl>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>