fix: NRE during disposal of stubbed MqttTransport

[vzam]

When external transports are disabled, the current implementation of MqttTransport would throw a NullReferenceException upon disposal. This PR aims at fixing that.

Minimal reproduction of issue:

using Wolverine;
using Wolverine.MQTT;

var host = Host.CreateDefaultBuilder()
    .UseWolverine(opts =>
    {
        opts.UseMqttWithLocalBroker();
        opts.StubAllExternalTransports();
    });

var app = host.Build();

await app.StartAsync();

app.Dispose();