ipn/ipnlocal: only call serve handler if non-nil

return early if handler is nil. Go ahead and return the error from
handler, though in this case the caller isn't doing anything with it
(which has always been the case).

Updates #10177
Updates #10251

Signed-off-by: Will Norris <will@tailscale.com>

ipn/ipnlocal/serve.go

@@ -93,9 +93,9 @@ func (b *LocalBackend) newServeListener(ctx context.Context, ap netip.AddrPort,
 			if handler == nil {
 				b.logf("[unexpected] local-serve: no handler for %v to port %v", srcAddr, ap.Port())
 				conn.Close()
+				return nil
 			}
-			handler(conn)
-			return nil
+			return handler(conn)
 		},
 		bo: backoff.NewBackoff("serve-listener", logf, 30*time.Second),
 	}