Check for null or empty message list from session.ReceiveBatchAsync() before calling session.CompleteBatchAsync()

```
…
newMessages = await Utils.ExecuteWithRetries(() => session.ReceiveBatchAsync(PrefetchCount),
session.SessionId, "Receive Tracking Message Batch",
MaxRetriesServiceBus,
IntervalBetweenRetriesSecs);

….
…

IEnumerable lockTokens = newMessages.Select(m => m.LockToken);
Utils.SyncExecuteWithRetries(() =>
{
session.CompleteBatch(lockTokens);
return null;
},
```

The complete batch throws an error if lockTokens is an empty list.