Skip SLA check only if SLA is None (apache#14064)

Users have to provide SLA as timedelta. If not provided it should
be a default None value.

This change will result in run time errors if other type than
timedelta is passed as SLA. With this change spotting apache#14056
would be easier.

airflow/jobs/scheduler_job.py

@@ -421,8 +421,11 @@ def manage_slas(self, dag: DAG, session: Session = None) -> None:
         ts = timezone.utcnow()
         for ti in max_tis:
             task = dag.get_task(ti.task_id)
-            if not isinstance(task.sla, timedelta):
-                continue
+            if task.sla and not isinstance(task.sla, timedelta):
+                raise TypeError(
+                    f"SLA is expected to be timedelta object, got "
+                    f"{type(task.sla)} in {task.dag_id}:{task.task_id}"
+                )
 
             dttm = dag.following_schedule(ti.execution_date)
             while dttm < timezone.utcnow():