fixes #19969 - consider subnet6 when determining vlanid

You can have a network interface with an ipv6 subnet configured but
without an ipv4 subnet; in this case, the function `vlanid` for
`interface` would throw an error. This PR fixes that error.

This fix will test for `nil` on `subnet` and fall back to `subnet6`.
If both subnets are absent, we return an empty string. It also takes
into account blank vlanids