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

Change-Id: Ief475a1bf6e65b4786e8ee9c025b204f143a30e0
Reviewed-on: https://gerrit.libreoffice.org/7079
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>