Bug 34883: Stop patron expiry date being set to NULL during import

Bug 34883: Stop patron expiry date being set to NULL during import

A regression has been identified whereby an empty field in the dateexpiry field in a patron import file will cause the patron's expiry date to be set to NULL. This patch addresses this by checking for an empty field and using the existing expiry date if one is found.

Test plan:
1) Setup a csv with column headers:
    surname	firstname	branchcode	categorycode	cardnumber	dateenrolled dateexpiry
2) Add values:
    Acosta	Edna	CPL	PT	23529001000463	02/01/2013
3) Leave the dateexpiry column blank
4) Check Edna and make a note of her patron expiry date
5) Run the import_patrons.pl script with the following flags:
    a) --file <filepath_for_your_csv_file>
    b) --matchpoint cardnumber
    c) --confirm
    d) --overwrite
6) Check Edna, note her expiry date is now set to NULL
7) Manually edit Edna's expiry date to be reset to what it was before you ran the script
8) Apply the patch and restart_all
9) Repeat step 5
10) Check Edna, this time her expiry date should be the same as the value you set it to in step 7
11) Sign off!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>