Fix nil panic (#5153)

<!-- Describe what has changed in this PR -->
**What changed?**
Add nil check when using namespaceEntry

<!-- Tell your future self why have you made these changes -->
**Why?**
nil panic found at edge case

<!-- 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**
no

<!-- Is this PR a hotfix candidate or require that a notification be
sent to the broader community? (Yes/No) -->
**Is hotfix candidate?**
no

service/history/replication/raw_task_converter.go

@@ -116,8 +116,15 @@ func (c *SourceTaskConverterImpl) Convert(
 	if clientShardID != c.clientShardKey.ShardID {
 		return nil, nil
 	}
+	var ctx context.Context
+	var cancel context.CancelFunc
+
+	if namespaceEntry != nil {
+		ctx, cancel = newTaskContext(namespaceEntry.Name().String())
+	} else {
+		ctx, cancel = context.WithTimeout(context.Background(), applyReplicationTimeout)
+	}
 
-	ctx, cancel := newTaskContext(namespaceEntry.Name().String())
 	defer cancel()
 	replicationTask, err := c.historyEngine.ConvertReplicationTask(ctx, task)
 	if err != nil {