tstime/mono: make json.Unmarshal of a zero time.Time yield a zero Time

This was the proximate cause of #2579.
#2582 is a deeper fix, but this will remain
as a footgun, so may as well fix it too.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tstime/mono/mono.go

@@ -121,6 +121,10 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 	if err != nil {
 		return err
 	}
+	if tt.IsZero() {
+		*t = 0
+		return nil
+	}
 	*t = Now().Add(-time.Since(tt))
 	return nil
 }