net/dnscache: fix case where Resolver could return zero IP with single IPv6 address

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The controlhttp dialer with a ControlDialPlan IPv6 entry was hitting a
case where the dnscache Resolver was returning an netip.Addr zero
value, where it should've been returning the IPv6 address.

We then tried to dial "invalid IP:80", which would immediately fail,
at least locally.

Mostly this was causing spammy logs when debugging other stuff.

Updates tailscale/corp#32534

Change-Id: If8b9a20f10c1a6aa8a662c324151d987fe9bd2f8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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500 Internal Server Error

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