How to select NOT NULL columns from DB2
According to the IBM DB2 UDB for iSeries SQL Reference
V5R3  documents, the SYSCOLUMNS view contains an IS_NULLABLE column.  

DB2 for z/OS 10 has a NULLS column that indicates if a column is nullable in SYSIBM.SYSCOLUMNS table

Potentially, SYSCAT.COLUMNS may have the IS_NULLABLE or NULLS column.

I'm not a DB2 expert, so this may or may not be applicable to your system.