[FOLLOWUP][BUGFIX] Remove hardcoded hidden column

Use processedTca instead of $GLOBALS access and improve
validation to avoid notice level errors.

Change-Id: I9f2f2a455875e509c16cad17e29e629e8b21fe6d
Resolves: #75647
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/47892


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>