Bug 27968: Fix framework import if isurl has an incorrect value

Bug 27968: Fix framework import if isurl has an incorrect value

If isurl is set to undef in the DB, it will be exported as an empty
string, which is an invalid value for isurl (int)

Incorrect integer value: '' for column 'isurl'

Test plan:
Export framework structure in CSV and ODS, then reimport it and check that
all the subfields are important correctly

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 01f0e04172432e28f37a94bf6df51a3fd6cdc960)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>