[BUGFIX] CONTENT cObj: Set 'select.' to array if empty

Ensure that the 'select.' configuration is an array
to avoid PHP errors.

Resolves: #72817
Releases: master, 7.6
Change-Id: I749fec996759883d78a2566c9d6b668ed4c7955e
Reviewed-on: https://review.typo3.org/46099


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>