Change the type of the `IPv4` and `IPv6` members in the `nodeData`
struct to be `netip.Addr` instead of `string`.
We were previously calling `String()` on this struct, which returns
"invalid IP" when the `netip.Addr` is its zero value, and passing this
value into the aforementioned attributes.
This caused rendering issues on the frontend
as we were assuming that the value for `IPv4` and `IPv6` would be falsy
in this case.
The zero value for a `netip.Addr` marshalls to an empty string instead
which is the behaviour we want downstream.
Updates https://github.com/tailscale/tailscale/issues/14568
Signed-off-by: Mario Minardi <mario@tailscale.com>
0b4ba4074f