Bug 11912: fix problem where GetMarcISBN wrongly prepends a space to ISBNs

Bug 11912: fix problem where GetMarcISBN wrongly prepends a space to ISBNs

This patch makes the logic inside GetMarcISBN simpler and
fixes the issue.

To test:
- Run the regression tests:
  prove -v t/db_dependent/Biblio.t
=> FAIL
- Apply the patch
- Run:
  prove -v t/db_dependent/Biblio.t
=> SUCCESS
- Verify that opac-detail.pl and catalogue/detail.pl look as usual regarding ISBN
- Sign off

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit c4900dc448aa029749ab27f98b59d1be6eb8bb14)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>