BE: RBAC: Add cluster list validation (#421)

Author: Haarolean

api/src/main/java/io/kafbat/ui/model/rbac/Role.java

@@ -1,5 +1,6 @@
 package io.kafbat.ui.model.rbac;
 
+import com.google.common.base.Preconditions;
 import java.util.List;
 import lombok.Data;
 
@@ -12,6 +13,7 @@ public class Role {
   List<Permission> permissions;
 
   public void validate() {
+    Preconditions.checkArgument(!clusters.isEmpty(), "Role clusters cannot be empty");
     permissions.forEach(Permission::transform);
     permissions.forEach(Permission::validate);
   }