In short, Sybase ASE default when creating a table column without specifying NULL or NOT NULL seems to differ from the standard behavior where NULL is the default (see W3 Schools). This behavior is controlled by the server, and the default setting varies with the type of the server and how it is configured.
This may cause problems if you use DbVisualizer to generate scripts from a table since DbVisualizer assumes that NULL is the default.
You should be able to override this by setting the JDBC property allow_nulls_by_default as described in Configuring Connection Properties - Driver Properties and the jConnect documentation. Alas, this is apparently not always possible; it may depend on what driver/driver version and server/server version you run. If there is no way to override this by configuring the driver, you may need to discuss this with your DBA.