Bug 33900: Fix handling of due_digest

Test plan, on k-t-d
1) Go to 'my account'
2) On 'Patron messaging preferences', click 'Edit'
3) On the 'Item due' row, check the 'Email' and 'Digests only' checkboxes and save
4) On the top search bar, press 'Check out' and enter '42' (koha user cardnumber)
5) On the checkout input bar, enter a barcode e.g. 39999000001372 and press checkout
6) Run the following query to force the due_date to be equal to 'todays' date:
NOTE: change the YYYY-MM-DD below to whatever day it is you're running this test plan

UPDATE issues SET date_due = '2023-06-05 23:59:00' where issue_id = 1;

7) Run the cronjob:
./koha/misc/cronjobs/advance_notices.pl -c --digest-per-branch

8) Notice it hangs, it's stuck on an infinite loop on the 2nd while cycle after if ()
9) Cancel the cronjob script, apply patch
10) Run the cronjob script again (step 7), notice it finishes
11) Access patron's notices:
/cgi-bin/koha/members/notices.pl?borrowernumber=51

Check that the 'Item due reminder' notice is there.

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

misc/cronjobs/advance_notices.pl

@@ -419,7 +419,7 @@ if ($digest_per_branch) {
     while (my ($branchcode, $digests) = each %$due_digest) {
         send_digests({
             sth => $sth_digest,
-            digests => $due_digest,
+            digests => $digests,
             letter_code => 'DUEDGST',
             branchcode => $branchcode,
             message_name => 'item_due',