Check for label presence in the validation, don't validate labels on simple role setup

lib/kamal/configuration/validator.rb

@@ -170,6 +170,8 @@ class Kamal::Configuration::Validator
     end
 
     def validate_labels!(labels)
+      return true if labels.blank?
+
       with_context("labels") do
         labels.each do |key, _|
           with_context(key) do

lib/kamal/configuration/validator/accessory.rb

@@ -6,7 +6,7 @@ class Kamal::Configuration::Validator::Accessory < Kamal::Configuration::Validat
       error "specify one of `host`, `hosts`, `role`, `roles`, `tag` or `tags`"
     end
 
-    validate_labels!(config["labels"]) if config.key?("labels")
+    validate_labels!(config["labels"])
 
     validate_docker_options!(config["options"])
   end

lib/kamal/configuration/validator/role.rb

@@ -2,12 +2,12 @@ class Kamal::Configuration::Validator::Role < Kamal::Configuration::Validator
   def validate!
     validate_type! config, Array, Hash
 
-    validate_labels!(config["labels"]) if config.key?("labels")
 
     if config.is_a?(Array)
       validate_servers!(config)
     else
       super
+      validate_labels!(config["labels"])
       validate_docker_options!(config["options"])
     end
   end