Bug 25805: Return empty strings instead of undef in C4::SIP::ILS::Item::hold_patron_name

Bug 25805: Return empty strings instead of undef in C4::SIP::ILS::Item::hold_patron_name

This bug is basically the same as bug 24966, but for hold_patron_name instead of hold_patron_bcode.
The subroutine hold_patron_bcode should always return an empty string, not undef.

Test Plan:
1) Using the SIP cli emulator, checkin an item that is not checked out
2) Note the DA field contains someting like "C4::SIP::SIPServer=HASH(0x88175c8)"
   The hex number will almost certainly be different from this example
3) Apply this patch
4) Restart the SIP server
5) Run the SIP checkin again
6) Note the DA field is no longer present!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>