Make "name" column nullable in workflow operations

The "name" column is now unused and the code is always inserting an
empty string. While this works with most databases, Oracle complains
because an empty string is equivalent to null.

To fix this, the column definition is changed to allow null values now.

Also added some logging in case of database exceptions, because without
this nothing would be logged to detect the above problem.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>