Bug 23309: Use Koha::MarcSubfieldStructures to create to subfields

Bug 23309: Use Koha::MarcSubfieldStructures to create to subfields

It will fix the insert of new subfields with SQL strict modes

DBD::mysql::st execute failed: Incorrect integer value: '' for column
'hidden' at row 1 [for Statement "

Test plan:
- Turn the config strict_sql_modes ON
- Add a new subfield to a field

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>