This patch introduces regression tests for as_marc_field. It makes sure
there are empty subfields and then uses List::MoreUtils' all to verify
all generated MARC subfields contain something (i.e. the undef ones are
skipped).
Signed-off-by: David Nind <david@davidnind.com> Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org> Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com> Signed-off-by: Joy Nelson <joy@bywatersolutions.com>