code: private String ifNull(int rowIndex){
  
          if(params.length != 2){
              throw new IllegalArgumentException("Wrong number of Parameters for if null expression");
          }
  
          final RecordList.Record currentRecord = recordList.getRecords().get(rowIndex);
          final String field1 = currentRecord.getValueByFieldName(params[0]);
  
          if (!field1.isEmpty()){
              return field1;
          }
          final String field2 = currentRecord.getValueByFieldName(params[1]);
          return field2;
      }
docs: Given 2 fields, if the first is null then return the second field
  In the config file use:
  fieldName="func(IFNULL(field1,field2))"