Bug 26612: Error during web install for it-IT translation

Bug 26612: Error during web install for it-IT translation

Patch changes some of the values in the sql statement that inserts 307a
unimarc row into the marc_subfield_structure which created an SQL error
on installation.

The fields changed are:
- isurl now 0 instead of NULL,
- hidden now -5 instead of NULL,
- seealso empty string instead of NULL,
- link empty string instead of NULL.

These field values are now similar to the ones inserted for 306a or 308a.
The value that caused the SQL error was using NULL for hidden (due to
the NOT NULL constraint in the table definition).

I have tested the install with this change and found no issues.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 7c74c6636f779bbbaceee74f8704df9ec625815d)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>