MqttConnectionHandler race condition

The initial state of `MqttConnectionHandler` can result in a `NullReferenceException` in the time between generic host starup and "full" startup. In my test environment this occurs consistently. For a workaround, I explicitly pull and inject the dependencies by calling `StartAsync` after building and prior to starting the host.

As I understand it, the connection should be terminated rather than remain unhandled in an invalid state.

```diff
public override async Task OnConnectedAsync(ConnectionContext connection)
{
...
var clientHandler = ClientHandler;
if (clientHandler != null)
{
await clientHandler(adapter).ConfigureAwait(false);
}
+ else
+ {
+ connection.Abort();
+ }
}
```

_Initially called out by @xljiulang in https://github.com/dotnet/MQTTnet/issues/2102#issuecomment-2475868014_

Edited for clarity and to sync up to current master