On RST with empty response, transaction.response could be None

[phc-1020]

During RST command, if the transaction response is empty, the field transaction.response may still be None. However, it is referenced at line 174 in file coapthon/layers/messagelayer.py, function receive_empty():
elif not transaction.response.acknowledged:
Suggestion: first check if transaction.response is not None:
elif (transaction.response is not None) and (not transaction.response.acknowledged):
This is exactly what is done in the case of the ACK command, at line 169 of the same file.