Bug 30571: Table z3950servers: Make host, syntax and encoding NOT NULL

Bug 30571: Table z3950servers: Make host, syntax and encoding NOT NULL

This is not a sync issue between structure and db revs.
It just seems a logical move.
Note that the interface for long does not allow these fields to be empty.

Test plan:
Run dbrev.
Add a new Z3950/SRU server. Verify if it works as expected.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>