Bug 14480: Silences warn from editing patron

 Bug 14480: Silences warn from editing patron

Argument "" isn't numeric in numeric lt (<)

TEST PLAN
---------
1) Go to any patron in staff client
2) Click 'Edit'
3) Check error logs
   -- warnings should exist.
4) Apply patch
5) Reload page
   -- warnings should not be triggered.
6) Add restriction and save
7) Go back to edit screen
   -- restriction should be shown
8) Remove restriction and save
9) Go back to edit screen
   -- Should say "Patron is currently unrestricted"
10) Run koha qa test tools.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>