Transaction Frame commands allow null transaction ID

[whikloj]

In Stomp\Protocol\Protocol the methods for transaction BEGIN, COMMIT & ABORT commands all allow null for the transaction ID.
https://github.com/stomp-php/stomp-php/blob/master/src/Protocol/Protocol.php#L161
https://github.com/stomp-php/stomp-php/blob/master/src/Protocol/Protocol.php#L174
https://github.com/stomp-php/stomp-php/blob/master/src/Protocol/Protocol.php#L187

This seems to violate the Stomp specification which says

The transaction header is REQUIRED, and the transaction identifier will be used for SEND, COMMIT, ABORT, ACK, and NACK frames to bind them to the named transaction.

and

The transaction header is REQUIRED and MUST specify the identifier of the transaction to (commit|abort).

https://stomp.github.io/stomp-specification-1.2.html#BEGIN
https://stomp.github.io/stomp-specification-1.2.html#COMMIT
https://stomp.github.io/stomp-specification-1.2.html#ABORT

Either the $transactionId should be required or the method should throw an Exception if the $transactionId is null.