Bug 20537: Added checks to remove warning from overdue_notices.pl

Bug 20537: Added checks to remove warning from overdue_notices.pl

When executing overdue_notices.pl on borrowers that lack a surname, we
see this error:
Use of uninitialized value in concatenation (.) or string at
/usr/share/koha/bin/cronjobs/overdue_notices.pl line 575.

This patch fixes this issue by setting the $borr variable
based on the information that has been defined.

To test:
1) Create a borrower and set its surname to null
2) Checkout an item to the borrower and set it to be overdue
3) Navigate to kohaclone/misc/cronjobs and enter a koha-shell
4) Run the script: ./overdue_notices.pl
5) Observe the error appears
6) Apply the patch
7) Repeat steps 3-4
8) Observe the error is gone
9) Sign off!

Sponsored-by: Catalyst IT
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>