[BUGFIX] Warnings in DataHandler on undefined array indexes

Accessing array indexes that actually do not exist, results
in a PHP warning. This behavior is caught by using isset()
or empty() on NULL values, but is not if using the function
array_key_exists(). Thus, an explicit check on NULL is added.

Resolves: #69948
Releases: master
Change-Id: I4fc9bb5ef9e8ff47ac199e0a7c02dd88ff6c1660
Reviewed-on: http://review.typo3.org/43423


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>