In sys.db.Manager.doUpdate(), if cache is null, update all fields

Fixes glitch where:

- existing record has non-null value
- you have a sys.db.Object with no cache (created manually, via remoting etc)
- set the value to null, try to update
- previous value remains.

Please note the private method isBinary() is now no longer used and could
be removed, though it may be useful in future.