Bug 14075: Undefined value creates noisy warns in C4::AuthoritiesMarc

Bug 14075: Undefined value creates noisy warns in C4::AuthoritiesMarc

This match sets $sortby (previously undefined value) as an empty string to get rid of the warns.

To test:

1) Go to a URL such as http://localhost:8080/cgi-bin/koha/opac-authorities-home.pl?op=do_search&type=opac&operator=contains&value=a&marclist=any&and_or=and
2) Notice the warns in the error log
3) Apply patch
4) Reload URL
5) Notice page still works but no warns in error log

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
NOTE: I would have done $sortby //= '';
      But this works too. :)

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>