v4l2src: implement "no signal" like we have for decklink sources

Currently, voctocore crashes with this error:

```
Fri 2025-04-18 18:14:21 CEST encoder96.lan.c3voc.de voctocore.py[445787]:    ERROR Pipeline: GStreamer pipeline element 'v4l2src0' signaled an error #9: Could not read from resource.
```

voctocore will not start unless the device is available.