[BUGFIX] Remove NOT NULL from optional text fields in system extensions

Removes NOT NULL requirement from TEXT database columns where the field is
not required to be filled in the backend. This aligns TCA definition and
database configuration, allowing to skip those fields when creating
records as MySQL doesn't allow default values for TEXT columns.

Resolves: #71952
Releases: master
Change-Id: Ic863899c6343a8e8c603a265c85dd906ad834568
Reviewed-on: https://review.typo3.org/45039


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>