Columns needs to be non-nullable for DB2

For DB2, we can not create constraints over nullable
columns. This patch makes some related changes to
035_event_uuid_to_id.py for Heat.

Change-Id: I6d251fa503848e03603a13ecb742d4915f0f6972
Closes-Bug: #1328045
Co-authored-by: Matt Riedemann <mriedem@us.ibm.com>
Merge "Columns needs to be non-nullable for DB2"