network-tunnel: Don't fail on Stop() of nil tunnel

Currently `source-oracle` does an unconditional `db.tunnel.Stop()`
during shutdown regardless of whether a network tunnel exists, but
the implementation of `Stop()` assumes that the tunnel must not be
nil. This disagreement could be resolved in either direction, but
it seemed cleaner to just make `Stop()` a no-op for nil receivers.

go/network-tunnel/network_tunnel.go

@@ -126,6 +126,9 @@ func (t *SshTunnel) Start() error {
 }
 
 func (t *SshTunnel) Stop() {
+	if t == nil {
+		return
+	}
 	if t.Cmd != nil {
 		// Using the negative pid signals kill to a process group.
 		// This ensures the children of the process are also killed