Ensure valid actor ID in actorExists method

Updated the actorExists method to validate that the actor ID is not blank and adheres to ObjectId format. This prevents invalid or malformed IDs from causing issues during runtime.

Author: renczesstefan

application-engine/src/main/java/com/netgrif/application/engine/workflow/service/TaskService.java

@@ -865,7 +865,7 @@ private List<String> getExistingActors(ActorListFieldValue actorListFieldValue)
     }
 
     private boolean actorExists(ActorFieldValue actorFieldValue) {
-        if (actorFieldValue == null || actorFieldValue.getId() == null) {
+        if (actorFieldValue == null || actorFieldValue.getId() == null || actorFieldValue.getId().isBlank() || !ObjectId.isValid(actorFieldValue.getId())) {
             return false;
         }
         AbstractUser user = userService.findById(actorFieldValue.getId(), actorFieldValue.getRealmId());