[BUGFIX] use array_key_exists() to deal correctly with NULL values

Resolves: #68606
Releases: master
Change-Id: I22fbd200514324eca4a62b87d2a3018e87c8a85e
Reviewed-on: http://review.typo3.org/42081


Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
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: Tomas Norre Mikkelsen <tomasnorre@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>