fix panic when replicationTask is nil (#6335)

## What changed?
check replicationTask to avoid panic

## Why?
In some cases, ConvertReplicationTask may return nil replicationTask. We
need to check that when setting RawTaskInfo.

## How did you test it?
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test? Checked in staging env? -->

## Potential risks
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change to production? -->

## Documentation
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stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->

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service/history/replication/raw_task_converter.go

@@ -108,11 +108,13 @@ func (c *SourceTaskConverterImpl) Convert(
 	if err != nil {
 		return nil, err
 	}
-	rawTaskInfo, err := c.serializer.ParseReplicationTaskInfo(task)
-	if err != nil {
-		return nil, err
+	if replicationTask != nil {
+		rawTaskInfo, err := c.serializer.ParseReplicationTaskInfo(task)
+		if err != nil {
+			return nil, err
+		}
+		replicationTask.RawTaskInfo = rawTaskInfo
 	}
-	replicationTask.RawTaskInfo = rawTaskInfo
 	return replicationTask, nil
 }