[SalesForce] How to make approval comments mandatory

I have a trigger to make rejection comments mandatory. I have many records that have already been approved/rejected without comments. When a user attempts to reject a record that has already been approved or reject AND HAS NO comments, they receive an error (RequireRejectionComment: execution of BeforeUpdate caused by: System.ListException: List index out of bounds: 0: Trigger.RequireRejectionComment: line 29, column 1).

trigger RequireRejectionComment on carrier__c (before update) {

  Map<Id, carrier__c> rejectedStatements = new Map<Id, carrier__c>{};

  for(carrier__c car: trigger.new)
  {
    carrier__c oldcar = System.Trigger.oldMap.get(car.Id);

    if (oldcar.Approval_Status__c != 'Rejected' 
    && car.Approval_Status__c == 'Rejected')
    { 
        rejectedStatements.put(car.Id, car);
    }
  }
  for (ProcessInstance pi : [SELECT TargetObjectId, 
                              (  
                                 SELECT Id, StepStatus, Comments 
                                 FROM Steps
                                 WHERE StepStatus = 'Rejected'
                                 ORDER BY CreatedDate DESC
                                 LIMIT 1 
                              )
                               FROM ProcessInstance
                               WHERE TargetObjectId In 
                                 :rejectedStatements.keySet()
                               ORDER BY CreatedDate DESC
                              ])
  {     
      ***if ((pi.Steps[0].Comments == null || pi.Steps[0].Comments.trim().length() == 0))***
      {
          rejectedStatements.get(pi.TargetObjectId).addError(
          'Please enter a rejection comment!');
      }
  }
}

Your guidance is much appreciated.