Bug 28293: (bug 20443 follow-up) Fix wrong key in Patrons::Import->generate_patron_at...

Bug 28293: (bug 20443 follow-up) Fix wrong key in Patrons::Import->generate_patron_attributes

It generates warnings
Use of uninitialized value in string comparison (cmp) at /kohadevbox/koha/Koha/Patrons/Import.pm line 550, <$fh> line 2.

And does not sort correctly (no fallback on the attribute's value for repeatable attributes)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 2da36e3f175de1fcf23bdfbc96b8053f3a0a7d26)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>