Bug 35181: Don't pass undef to header

Bug 35181: Don't pass undef to header

It seems passing an undef value causes the headers to be malformed.

To test:
1 - Visit http://localhost:8081/cgi-bin/koha/svc/bib/303
2 - Note 'Invalid-metadata' at top of page, as well as other headers
3 - Apply patch, restart all
4 - Visit http://localhost:8081/cgi-bin/koha/svc/bib/303
5 - Page should now appear as xml
6 - Edit record 303, or any record with valid metadata in the advanced cataloging editor
7 - Confirm there is no error or alert
8 - Edit record 369 in the advanced cataloging editor
9 - Confirm there is an alert

Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>