control/controlclient: fix panic regression from earlier load balancer hint header

In the recent 20e9f3369 we made HealthChangeRequest machine requests
include a NodeKey, as it was the oddball machine request that didn't
include one. Unfortunately, that code was sometimes being called (at
least in some of our integration tests) without a node key due to its
registration with health.RegisterWatcher(direct.ReportHealthChange).

Fortunately tests in corp caught this before we cut a release. It's
possible this only affects this particular integration test's
environment, but still worth fixing.

Updates tailscale/corp#1297

Change-Id: I84046779955105763dc1be5121c69fec3c138672
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
(cherry picked from commit 8444937c89)

control/controlclient/direct.go

@@ -1719,7 +1719,10 @@ func (c *Direct) ReportHealthChange(sys health.Subsystem, sysErr error) {
 		// Don't report errors to control if the server doesn't support noise.
 		return
 	}
-	nodeKey := c.GetPersist().PublicNodeKey()
+	nodeKey, ok := c.GetPersist().PublicNodeKeyOK()
+	if !ok {
+		return
+	}
 	req := &tailcfg.HealthChangeRequest{
 		Subsys:  string(sys),
 		NodeKey: nodeKey,