crun should never checkpoint the netns

Basically the same as 2a0947e601c354f0c63a3802f8084db1b23e1851 but this made the exception to checkpoint the netns when the netns path is empty in the runtime spec. This works only for the case where podman creates a netns in advance but this is not always the case, i.e. when a custom userns is used (which also doesn't work in crun right now but this is a different issue #1207).

The problem now is that I want to consolidate the network setup code in podman https://github.com/containers/podman/pull/18468 to only use one setup path instead of two for with and without userns. So going forward I always want to let the runtime create the netns (empty netns path in config) and after the create call configure the netns in podman. This works just fine except for the checkpoint/restore case. On restore criu tries to restore the netns which fails:

```
(00.048662) 1: Try to restore a link 10:2:eth0
(00.048676) 1: Restoring link eth0 type 2
(00.048691) 1: Restoring netdev eth0 idx 2
(00.048705) 1: Restore ll addr (8e:../6) for device
(00.048712) 1: Error (criu/net.c:1462): Unknown peer net namespace
(00.063166) 1: Error (criu/libnetlink.c:54): -16 reported by netlink: Device or resource busy
(00.063205) 1: Error (criu/net.c:1816): Can't restore link: -16
(00.063270) 1: Error (criu/util.c:1411): Can't wait or bad status: errno=0, status=65280
(00.064717) Error (criu/cr-restore.c:2536): Restoring FAILED.
```

The same commands work just fine with runc because they always ignore the netns. https://github.com/opencontainers/runc/pull/1840/commits/8187fb740c202f5d29f1717bb933143c10dae8a1

So my ask is to always ignore the netns to match runc behavior and allow poman to work correctly.