Bug 12674: GetMarcISSN should not return empty ISSN

Bug 12674: GetMarcISSN should not return empty ISSN

This patch makes GetMarcISSN test for empty subfield before pushing to the
result array.

To test:
- Run the regression test
=> FAILS for all MARC flavours
- Apply the patch
- Run the regression test
=> SUCCESS: tests pass
- Sign off

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>