net/dns: check if Manager.knobs isn't nil before using it

Updates corp#15802

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

net/dns/manager.go

@@ -296,6 +296,9 @@ func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig
 }
 
 func (m *Manager) disableSplitDNSOptimization() bool {
+	if m.knobs == nil {
+		return false
+	}
 	return m.knobs.DisableSplitDNSWhenNoCustomResolvers.Load()
 }