Bug 23415: (follow-up) Fallback to 'days' in 'get_chargeable_units'

Signed-off-by: Tomás Cohen Arazi <tomascohen@theke.io>
Sponsored-by: OpenFifth <https://openfifth.co.uk/>

C4/Overdues.pm

@@ -346,6 +346,9 @@ sub get_chargeable_units {
 
     my $charge_units = 0;
     my $charge_duration;
+
+    $unit //= 'days';
+
     if ( $unit eq 'hours' ) {
         if ( C4::Context->preference('finesCalendar') eq 'noFinesWhenClosed' ) {
             my $calendar = Koha::Calendar->new( branchcode => $branchcode );