/dev/null is supposed to be added to the device cgroup unconditionally: https://github.com/snapcore/snapd/blob/master/cmd/snap-confine/udev-support.c#L169
You can check to see if it is in the device cgroup by looking for c 1:3 rwm in /sys/fs/cgroup/devices/snap.name.cmd/devices.list.
If it isn’t there, we’d need to see why snap-confine isn’t adding the device. (cc @mvo5)