[BUGFIX] DBAL: Set allowNull for insert and update queries to TRUE

When DBAL is installed, it was not possible to save NULL values to database, but
TCA fields with mode=useOrOverridePlaceholder fields strongly requires this.
Without the possibility to store NULL to database, "useOrOverridePlaceholder"
would always override the original value.

Resolves: #63991
Releases: master, 6.2
Change-Id: I23cd221431ba9d00e588942bc7c7030e9af805dc
Reviewed-on: http://review.typo3.org/36545


Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>