The AlwaysOn policy can be applied by (*LocalBackend).applySysPolicy, flipping WantRunning from false to true
before (*LocalBackend).Start() has been called for the first time and set a control client in b.cc. This results in a nil
pointer dereference and a panic when setPrefsLockedOnEntry applies the change and calls controlclient.Client.Login().
In this PR, we fix it by only doing a login if b.cc has been set.
Updates #14823
Signed-off-by: Nick Khyl <nickk@tailscale.com>