Bug 23077: Don't fill cardnumber with empty string

Bug 23077: Don't fill cardnumber with empty string

To test:
1 - Apply unit test patch only
2 - prove the tests, they fail
3 - Attempt to import the attached sample file
4 - First patron may be imported, second will give error (or both will error)
5 - If only one error, import agian, now both have error
6 - Apply this patch
7 - Prove tests, they pass!
8 - Import the sample file
9 - It succeeds!

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit cc60770c4eed3ab8144acd2382e2894f7b63fb37)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>