Dynamic level control should not fall back to Verbose when no server level is specified

[nblumhardt]

var levelSwitch = new LoggingLevelSwitch(LogEventLevel.Warning);

var logger = new LoggerConfiguration()
    .MinimumLevel.ControlledBy(levelSwitch)
    .WriteTo.Seq(..., controlLevelSwitch: levelSwitch)
    .CreateLogger();

In this scenario, if the server does not have a minimum level set for the corresponding API key, the switch should not be adjusted.

If the server had, but then removes, the level set for the key, then the fallback level should be Warning, not Verbose as it is today.