fix: add error checking in acl and poll

If aclPolicy is not defined, in updateAclPolicy, return an error.

acls.go

@@ -86,6 +86,10 @@ func (h *Headscale) UpdateACLRules() error {
 func (h *Headscale) generateACLRules() ([]tailcfg.FilterRule, error) {
 	rules := []tailcfg.FilterRule{}
 
+	if h.aclPolicy == nil {
+		return nil, errEmptyPolicy
+	}
+
 	machines, err := h.ListAllMachines()
 	if err != nil {
 		return nil, err

poll.go

@@ -95,6 +95,7 @@ func (h *Headscale) PollNetMapHandler(ctx *gin.Context) {
 	now := time.Now().UTC()
 
 	// update ACLRules with peer informations (to update server tags if necessary)
+	if h.aclPolicy != nil {
 		err = h.UpdateACLRules()
 		if err != nil {
 			log.Error().
@@ -103,7 +104,7 @@ func (h *Headscale) PollNetMapHandler(ctx *gin.Context) {
 				Str("machine", machine.Name).
 				Err(err)
 		}
-
+	}
 	// From Tailscale client:
 	//
 	// ReadOnly is whether the client just wants to fetch the MapResponse,