Bug 9201 OAI-PMH mapping value cannot be 0

Allow the value of a OAI set mapping to be 0 / empty. Delete the mapping if field or subfield are empty.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Now it is possible to add a value == 0 or an empty string.
Signed-off-by: Mason James <mtj@kohaaloha.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Merge branch 'bug_9201' into 3.12-master