resolved: suppress ifindex info in varlink JSON responses if zero

resolved: suppress ifindex info in varlink JSON responses if zero

If we don't have ifindex info, don't set the field for it.

We already do that for parsed IP address replies, let's do it for all
cases: it's a bit nicer to suppress the ifindex prop if it doesn't apply
than to pass it invalid.

This is the other side of #18482, i.e. fixes things so that the parser
doesn't get tripped up by this.

(This too makes a problem go away we should track down properly, i.e.
figure out how the ifindex got lost in
https://github.com/systemd/systemd/pull/17823#issuecomment-742439422 )