Unnecessary AuthSwitchRequest with old clients

When an old client connects with the PLUGIN_AUTH capability but it doesn't set the plugin name, MaxScale will send an AuthSwitchRequest packet which will cause an extra step to be taken in the authentication process. This can be observed in the following log message:

Client '...'@[...] is using an unsupported authenticator plugin ''. Trying to switch to 'mysql_native_password'. 

The correct thing to do would be to assume that an empty plugin is the same as mysql_native_password.