MT#62863 fix transform_billing_mappings() for debian/trixie

* _effective_start date is now correctly becomes 0 if undefined
  to address the general from_unixtime() change in MariaDB 11.5
  that requires a value.
* the fix is taken from:
  e935ac42 2025-04-29 MT#62676 fixes for mariadb 10.8/debian trixie [Rene Krenn]

Change-Id: I01cf96af3095fbf6e3cb9def9ddeb6c1f1883929

db_scripts/diff/15418.up

@@ -51,7 +51,7 @@ create procedure transform_billing_mappings() begin
               order by bm2.start_date desc limit 1) order by bm1.id asc;
           declare continue handler for not found set _mappings_done = true;
 
-          set _effective_start_time = (select unix_timestamp(if(_is_end,_t + 0.001,_t)));
+          set _effective_start_time = (select unix_timestamp(if(_is_end,_t + 1.001,_t + 1)) - 1);
           set _bm_ids = "";
           set _mappings_done = false;
           open mappings_cur;