Allow custom validation for all step types

[MAILPOET-4659]

mailpoet/lib/Automation/Engine/Control/RootStep.php

@@ -2,6 +2,7 @@
 
 namespace MailPoet\Automation\Engine\Control;
 
+use MailPoet\Automation\Engine\Data\StepValidationArgs;
 use MailPoet\Automation\Engine\Integration\Step;
 use MailPoet\Validator\Schema\ObjectSchema;
 
@@ -21,4 +22,7 @@ class RootStep implements Step {
   public function getSubjectKeys(): array {
     return [];
   }
+
+  public function validate(StepValidationArgs $args): void {
+  }
 }