odbc DBMetaData: NULL pattern equivalent to "%", not "%foo"

Change-Id: Ief475a1bf6e65b4786e8ee9c025b204f143a30e0
Reviewed-on: https://gerrit.libreoffice.org/7080
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>