control/controlclient: make Observer optional

As a baby step towards eventbus-ifying controlclient, make the
Observer optional.

This also means callers that don't care (like this network lock test,
and some tests in other repos) can omit it, rather than passing in a
no-op one.

Updates #12639

Change-Id: Ibd776b45b4425c08db19405bc3172b238e87da4e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/auto.go

@@ -117,7 +117,7 @@ type Auto struct {
 	logf          logger.Logf
 	closed        bool
 	updateCh      chan struct{} // readable when we should inform the server of a change
-	observer      Observer      // called to update Client status; always non-nil
+	observer      Observer      // if non-nil, called to update Client status
 	observerQueue execqueue.ExecQueue
 	shutdownFn    func() // to be called prior to shutdown or nil
 
@@ -170,9 +170,6 @@ func NewNoStart(opts Options) (_ *Auto, err error) {
 		}
 	}()
 
-	if opts.Observer == nil {
-		return nil, errors.New("missing required Options.Observer")
-	}
 	if opts.Logf == nil {
 		opts.Logf = func(fmt string, args ...any) {}
 	}
@@ -609,6 +606,11 @@ func (c *Auto) sendStatus(who string, err error, url string, nm *netmap.NetworkM
 		Err:     err,
 		state:   state,
 	}
+
+	if c.observer == nil {
+		return
+	}
+
 	c.lastStatus.Store(newSt)
 
 	// Launch a new goroutine to avoid blocking the caller while the observer