Refactor code to fix incorrect logger reference

In the DefaultTriggerScheduler.cs file, a wrong reference to _logger was corrected. It caused issues during attempts to log a warning when the 'cronExpression' parameter is empty. Now the correct local logger object is used.

src/modules/Elsa.Scheduling/Services/DefaultTriggerScheduler.cs

@@ -73,7 +73,7 @@ public async Task ScheduleAsync(IEnumerable<StoredTrigger> triggers, Cancellatio
 
             if (string.IsNullOrWhiteSpace(cronExpression))
             {
-                _logger.LogWarning("Cron expression is empty. TriggerId: {TriggerId}. Skipping scheduling of this trigger", trigger.Id);
+                logger.LogWarning("Cron expression is empty. TriggerId: {TriggerId}. Skipping scheduling of this trigger", trigger.Id);
                 continue;
             }