…ridges

The old assertion rejected sensible NixOS configs like this:
```
{
  networking.useDHCP = true; # this is the NixOS default
  networking.bridges.mybr.interfaces = [];
  networking.interfaces.mybr.useDHCP = false;
}
```
This config reflects the common use case of enabling DHCP for physical
NICs while adding a statically configured bridge.

Configs of this kind are now allowed.

All configs rejected by the new assertion were also
rejected by the old assertion.