Describe the bug

in kogito bpmn, I added a process variable (for example "travels" variable) of type Java class(which has only one variable of type List)
when I register the kogito generated proto in dataindex and try to run the application, I get InvalidSchemaException exception.

This exception goes away when I add more variable in the Java class along with List type variable.
But as per our requirement we want only one variable in that java class.

Expected behavior

data index should start without any exception.

Actual behavior

Data index start up is failing because of InvalidSchemaException: TravelsOrderBy must define one or more field.

How to Reproduce?

1. Create a Java class with one variable.
   public class Travels { private List travels; }

   public class Travel

   { private String travelId; }

1. Add a process variable in bpmn of type Travels class

1. generate proto and register it in data index.
   Sample:
   /Indexed/
   message Travels { options java_package = "some package"; repeated Travel travels = 1; }

1. Start the application

Refers to https://github.com/kiegroup/kogito-apps/issues/1422