don't say dashboard address if it's not actually available

```bash
~ python -m distributed.cli.dask_spec --spec '{"foo":{"cls":"dask.distributed.Scheduler"}}'

2023-07-12 09:15:06,973 - distributed.http.proxy - INFO - To route to workers diagnostics web server please install jupyter-server-proxy: python -m pip install jupyter-server-proxy
2023-07-12 09:15:06,975 - distributed.scheduler - INFO - State start
2023-07-12 09:15:06,992 - distributed.scheduler - INFO - Scheduler at: tcp://192.168.1.116:60558
2023-07-12 09:15:06,992 - distributed.scheduler - INFO - dashboard at: http://192.168.1.116:8787/status
```

But then

```bash
~ curl http://192.168.1.116:8787/status -v

* Trying 192.168.1.116:8787...
* Connected to 192.168.1.116 (192.168.1.116) port 8787 (#0)
> GET /status HTTP/1.1
> Host: 192.168.1.116:8787
> User-Agent: curl/7.87.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Server: TornadoServer/6.3.2
< Content-Type: text/html; charset=UTF-8
< Date: Wed, 12 Jul 2023 15:15:45 GMT
< Content-Length: 69
<
* Connection #0 to host 192.168.1.116 left intact
404: Not Found404: Not Found
```

Eventually I figured out that I needed to specify dashboard address:

```bash
~ python -m distributed.cli.dask_spec --spec '{"foo":{"cls":"dask.distributed.Scheduler", "opts": {"dashboard_address": ":8787"}}}'

2023-07-12 09:16:20,389 - distributed.http.proxy - INFO - To route to workers diagnostics web server please install jupyter-server-proxy: python -m pip install jupyter-server-proxy
2023-07-12 09:16:20,407 - distributed.scheduler - INFO - State start
2023-07-12 09:16:20,411 - distributed.scheduler - INFO - Scheduler at: tcp://192.168.1.116:60668
2023-07-12 09:16:20,411 - distributed.scheduler - INFO - dashboard at: http://192.168.1.116:8787/status
```

and then I can bring up the dashboard.

It seems like Dask shouldn't say `dashboard at: [...]` when that address doesn't actually work.