udev-builtin-net_id: ignore firmware_node/sun == 0

udev-builtin-net_id: ignore firmware_node/sun == 0

Since ID_NET_NAME_SLOT was introduced we ignore slot == 0
https://github.com/systemd/systemd/blob/0035597a30d120f70df2dd7da3d6128fb8ba6051/src/udev/udev-builtin-net_id.c#L139

Qemu sets _SUN to PCI_SLOT() for all NICs, so _SUN is not unique.
https://gitlab.com/qemu-project/qemu/-/issues/2530

In my tests with libvirt I can only set 'slot="0x00"' in interface definition,
so all NICs end up with _SUN == 0, and this commit is enough to avoid the issue.

Fixes 0a4ecc54cb9f2d3418b970c51bfadb69c34ae9eb