fix(conn): password can be set when username unset when mqttv5 #148
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MQTT-v5 allows sending a password without username
Which means username flag set
0and username not be present in CONNECT payload.And there is no necessary relationship between Username flag and Password flag.
see also:
https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901044