L2: RLC AM fix unexpected SDU cnf while receiving a NACK_SN with no vtA shift
When the RLC AM transmitter receives a NACK_SN with no effect on vtA update, there is unexpected SDU Data Cnf of whole Tx Window but without Stop of TPoll ReTx. It leads later to an assert in Tpoll Retx Expiry. The fix check that the last SN for SDU Cnf is not empty Seen in UE tests but fix also valid for eNB