Allow empty username

[Morksen]

We have a rabbitmq that uses authentication over oidc/oauth2. In that case, instead of username/password, we only have an accesskey set as the password to connect. Works fine in tools like postman, but MQTT-Explorer crashes.

"Error: Username is required to use password".

And even, if I confirm the error dialog, the error popup keeps reappearing every X seconds.