The Problem is, Could not list the output from the getRuleByCategory(). So you should create the list of DataIntegrityRule before paging.
Can you try this method to implement the listable DataIntegrityRules,
List<DataIntegrityRule> dataIntegrityRule = (ArrayList<DataIntegrityRule>)getDataIntegrityService().getRuleByCategory(ruleCategory);
return new NeedsPaging<>( dataIntegrityRule , requestContext);