Return nil instead of task discarded error if namespace is invalid lo…

…cally (#4678)

<!-- Describe what has changed in this PR -->
**What changed?**
* Return nil instead of task discarded error if namespace is invalid
locally

<!-- Tell your future self why have you made these changes -->
**Why?**
Returning task discarded error will make metrics confusing

<!-- How have you verified this change? Tested locally? Added a unit
test? Checked in staging env? -->
**How did you test it?**
N/A

<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->
**Potential risks**
N/A

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sent to the broader community? (Yes/No) -->
**Is hotfix candidate?**
N/A

service/history/timerQueueStandbyTaskExecutor.go

@@ -440,8 +440,8 @@ func (t *timerQueueStandbyTaskExecutor) processTimer(
 		return err
 	}
 	if !nsRecord.IsOnCluster(t.clusterName) {
-		// discard standby tasks
-		return consts.ErrTaskDiscarded
+		// namespace is not replicated to local cluster, ignore corresponding tasks
+		return nil
 	}
 
 	executionContext, release, err := getWorkflowExecutionContextForTask(ctx, t.cache, timerTask)

service/history/transferQueueStandbyTaskExecutor.go

@@ -508,8 +508,8 @@ func (t *transferQueueStandbyTaskExecutor) processTransfer(
 		return err
 	}
 	if !nsRecord.IsOnCluster(t.clusterName) {
-		// discard standby tasks
-		return consts.ErrTaskDiscarded
+		// namespace is not replicated to local cluster, ignore corresponding tasks
+		return nil
 	}
 
 	weContext, release, err := getWorkflowExecutionContextForTask(ctx, t.cache, taskInfo)