Fix null transaction id caused by VEvents without VGTID

[2022-02-03 03:03:56,883] INFO Stopping down connector (io.debezium.connector.common.BaseSourceTask)
	... 5 more
	at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1MessagesAvailable.runInContext(ClientCallImpl.java:700)
	at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1MessagesAvailable.runInternal(ClientCallImpl.java:715)
	at io.grpc.internal.DelayedClientCall$DelayedListener.onMessage(DelayedClientCall.java:447)
	at io.grpc.stub.ClientCalls$StreamObserverToCallListenerAdapter.onMessage(ClientCalls.java:465)
	at io.debezium.connector.vitess.connection.VitessReplicationConnection$1.onNext(VitessReplicationConnection.java:77)
	at io.debezium.connector.vitess.connection.VitessReplicationConnection$1.onNext(VitessReplicationConnection.java:99)
	at io.debezium.connector.vitess.connection.VStreamOutputMessageDecoder.processMessage(VStreamOutputMessageDecoder.java:58)
	at io.debezium.connector.vitess.connection.VStreamOutputMessageDecoder.handleBeginMessage(VStreamOutputMessageDecoder.java:114)
	at io.debezium.connector.vitess.VitessStreamingChangeEventSource.lambda$newReplicationMessageProcessor$0(VitessStreamingChangeEventSource.java:99)
	at io.debezium.pipeline.EventDispatcher.dispatchTransactionStartedEvent(EventDispatcher.java:282)
	at io.debezium.pipeline.txmetadata.TransactionMonitor.transactionStartedEvent(TransactionMonitor.java:148)
	at io.debezium.pipeline.txmetadata.TransactionMonitor.beginTransaction(TransactionMonitor.java:166)
	at org.apache.kafka.connect.data.Struct.put(Struct.java:203)
	at org.apache.kafka.connect.data.Struct.put(Struct.java:216)
	at org.apache.kafka.connect.data.ConnectSchema.validateValue(ConnectSchema.java:220)
Caused by: org.apache.kafka.connect.errors.DataException: Invalid value: null used for required field: "id", schema type: STRING
	... 3 more
	at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:123)
	at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
	at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInContext(ClientCallImpl.java:781)
	at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInternal(ClientCallImpl.java:802)
	at io.grpc.internal.ClientCallImpl.access$300(ClientCallImpl.java:69)
	at io.grpc.internal.ClientCallImpl.closeObserver(ClientCallImpl.java:616)
	at io.grpc.internal.DelayedClientCall$DelayedListener.onClose(DelayedClientCall.java:460)
	at io.grpc.internal.DelayedClientCall$DelayedListener.delayOrExecute(DelayedClientCall.java:427)
	at io.grpc.internal.DelayedClientCall$DelayedListener$3.run(DelayedClientCall.java:463)
	at io.grpc.stub.ClientCalls$StreamObserverToCallListenerAdapter.onClose(ClientCalls.java:478)
	at io.grpc.Status.asRuntimeException(Status.java:533)
Caused by: io.grpc.StatusRuntimeException: CANCELLED: Failed to read message.
	at java.base/java.lang.Thread.run(Thread.java:829)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at io.debezium.pipeline.ChangeEventSourceCoordinator.lambda$start$0(ChangeEventSourceCoordinator.java:108)
	at io.debezium.pipeline.ChangeEventSourceCoordinator.executeChangeEventSources(ChangeEventSourceCoordinator.java:139)
	at io.debezium.pipeline.ChangeEventSourceCoordinator.streamEvents(ChangeEventSourceCoordinator.java:172)
	at io.debezium.connector.vitess.VitessStreamingChangeEventSource.execute(VitessStreamingChangeEventSource.java:29)
	at io.debezium.connector.vitess.VitessStreamingChangeEventSource.execute(VitessStreamingChangeEventSource.java:78)

$ grpcurl -d '{ "vgtid": { "shard_gtids": [ { "keyspace": "test_keyspace", "shard": "80-", "gtid": "MySQL56/f84895a8-4eed-42ba-b500-469fd71cc08b:1-2499683:3415911-3415942:4415911-4415931" } ] } }' -plaintext xxxxxx:1234 vtgateservice.Vitess.VStream
{
  "events": [
    {
      "type": "BEGIN",
      "timestamp": "1637704151",
      "currentTime": "1643857685432455570"
    },
    {
      "type": "COMMIT",
      "timestamp": "1637704151",
      "currentTime": "1643857685432479353"
    }
  ]
}
{
  "events": [
    {
      "type": "BEGIN",
      "timestamp": "1643838860",
      "currentTime": "1643857685432505735"
    },
    {
      "type": "COMMIT",
      "timestamp": "1643838860",
      "currentTime": "1643857685432535479"
    }
  ]
}