Bug 14363: OAI-PMH: Handle records without marcxml

Bug 14363: OAI-PMH: Handle records without marcxml

Test plan:

1/ git checkout 3.18.x
2/ GET http://OPAC/cgi-bin/koha/oai.pl?verb=ListRecords&metadataPrefix=marcxml
3/ Take the first biblionumber from the results ($biblionumber)
4/ mysql> update biblioitems set marcxml=null where biblionumber=$biblionumber
5/ Repeat step 2 => XML error
6/ Apply patch
7/ Repeat step 2 => no errors

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
  Works on 3.20.x

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Fix works for 3.18.x and 3.20.x
3.22 is not impacted

Signed-off-by: Frédéric Demians <f.demians@tamil.fr>