[BUGFIX] 'eval' => 'null' field stays disabled

This is a regression fix, introduced with #75397

Resolves: #77287
Related: #75397
Releases: master, 7.6
Change-Id: I405646aee0acc6a67753c79ae4b3d3c5f8de1bed
Reviewed-on: https://review.typo3.org/49402


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Alexander Stehlik <alexander.stehlik@gmail.com>
Tested-by: Alexander Stehlik <alexander.stehlik@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>