ipn/ipnlocal,util/syspolicy/source: retain existing exit node when using auto exit node, if it's allowed by policy

In this PR, we update setExitNodeID to retain the existing exit node if auto exit node is enabled,
the current exit node is allowed by policy, and no suggested exit node is available yet.

Updates tailscale/corp#29969

Signed-off-by: Nick Khyl <nickk@tailscale.com>

util/syspolicy/source/test_store.go

@@ -154,6 +154,13 @@ func (s *TestStore) RegisterChangeCallback(callback func()) (unregister func(),
 	}, nil
 }
 
+// IsEmpty reports whether the store does not contain any settings.
+func (s *TestStore) IsEmpty() bool {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return len(s.mr) == 0
+}
+
 // ReadString implements [Store].
 func (s *TestStore) ReadString(key setting.Key) (string, error) {
 	defer s.recordRead(key, setting.StringValue)