[BUGFIX] Assure that $_POST is never set to null

Resolves: #86731
Releases: master
Change-Id: Ia48925cf98637d7c7b2ecfb071bc935508032dec
Reviewed-on: https://review.typo3.org/58682


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Daniel Sattler <sattler@b13.de>
Tested-by: Daniel Sattler <sattler@b13.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>