Pad blank rows to Flexigrid

$('#nav').flexigrid({
 url: '/Person/Management/List',
 dataType: 'json',
 colModel: [
  { display: 'Username', name: 'Username', width: 150, sortable: true, align: 'left' },
  { display: 'Firstname', name: 'Firstname', width: 150, sortable: true, align: 'left' },
  { display: 'Lastname', name: 'Lastname', width: 150, sortable: true, align: 'left' },
  { display: 'Favorite#', name: 'FavoriteNumber', width: 150, sortable: true, align: 'left' }
 ],
 buttons: [
  { name: 'Add', bclass: 'add', onpress: add },
  { separator: true },
  { name: 'Edit', bclass: 'edit', onpress: edit },
  { separator: true },
  { name: 'Delete', bclass: 'delete', onpress: del }
 ],
 singleSelect: true,
 sortname: 'Lastname',
 sortorder: 'asc',
 usepager: true,
 title: 'Persons',
 useRp: true,
 rp: 5, 
 rpOptions: [5, 10, 15, 20, 25, 40],
 showTableToggleBtn: true,
 width: 560,
 height: 'auto',
 preProcess: function (data) {                

    var rp = getFgRowsPerPage($('#nav'));
    for (i = data.rows.length; i < rp; ++i) {
        data.rows.push({ 'id': '', 'cell': ['', '', '', ''] });
    }
    return data;
 }
});

function getFgRowsPerPage(tbl) {
    return $('select[name=rp] option:selected', tbl.closest('.flexigrid')).val();
}

Sample output(the last two rows are from the for loop):