[BUGFIX] Allow NULL values in INSERT queries

Currently only UPDATE queries pass the $allowNull parameter to the
fullQuoteStr() method in the DatabaseHandler. To make the behavior of
both methods consistent and to allow NULL values during creation of
new records by TCEmain the $allowNull parameter is also set to TRUE for
INSERT queries.

Resolves: #53662
Releases: 6.2, 6.1, 6.0
Change-Id: I066b9880a557b6c9058fc15f467631f1313300f9