Fix issue unrefering null caps. Better solution than
```
if (src->caps)
gst_caps_unref (src->caps);
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6432>
Fix issue unrefering null caps. Better solution than
```
if (src->caps)
gst_caps_unref (src->caps);
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6432>