Ensure `msgUpdateClient` is not nil before appending to batch in `sendBatchMessages`

[LeTrongDat]

In the function sendBatchMessages, there is a logic to prepend mp.msgUpdateClient to the beginning of the batch. However, in cases where needsClientUpdate is false, msgUpdateClient will be nil. This can lead to appending a nil value into the batch, which is likely unintended and could cause issues downstream. It's necessary to introduce a check to ensure msgUpdateClient is not nil before appending it to the batch. This precaution would prevent potential errors or unexpected behavior during the message processing workflow.