Need help in highlighting NULL Values

Hi...
I need a condition in my webpage that if the LOGIN fields is NULL.
That row will be highlighted in RED.
i try this code but it did not work...

Code:

$LOGIN = $DTR->fields['LOGIN'];
if($LOGIN == NULL){
 "<td style='background-color:RED'>";
//echo '<style = background-color: RED;>'.$DTR.'</style>';
} 

I have this code:

Code:

<?php
include 'config.php';
$currentEmpID = $_SESSION['empID']; 
 
 if(!isset($_POST['Regsubmit_'])){ 
     
    $DATE1 = $_GET['Regfirstinput'];
    $DATE2   = $_GET['Regsecondinput'];

 $sql = "SELECT a.EMP_NO, CONCAT(LNAME, ', ' , FNAME, ' ', MI) AS FULLNAME, a.LOGIN, a.LOGOUT
 FROM $ATTENDANCE.employee_attendance AS a LEFT JOIN $ADODB_DB.employment em ON a.EMP_NO = em.EMP_NO 
 LEFT JOIN $ADODB_DB.personal p ON em.EMP_ID = p.EMP_ID WHERE DATE(LOGIN) BETWEEN '$DATE1' AND '$DATE2' 
 AND em.STATUS IN ('Reg Operatives', 'Reg Staff ') OR LOGIN IS NULL";  
    $DTR = $conn3->GetAll($sql);
    
$LOGIN = $DTR->fields['LOGIN'];
if($LOGIN == NULL){
 "<td style='background-color:RED'>";
//echo '<style = background-color: RED;>'.$DTR.'</style>';
}   
$smarty->assign('attendance', $DTR);
 }
 $smarty->display('header_att.tpl');
 $smarty->display('RegAttendance.tpl');
 $smarty->display('footer.tpl');
  
?> 

Code:

<div id="attendance" style="width: 580px; height: 420px; overflow: auto; padding: 5px; top:95px">
<fieldset>
<legend>Employee Attendance</legend>
<table border="1">
<tr>
<td colspan="2" style="text-align:center">Employee No</td>
<td colspan="2" style="text-align:center">Employee Name</td>
<td colspan="2" style="text-align:center">Time In</td>
<td colspan="2" style="text-align:center">Time Out</td>
</tr>
{section name=att loop=$attendance}
  <tr>
    <td colspan="2">{$attendance[att].EMP_NO}</td>
    <td colspan="2">{$attendance[att].FULLNAME}</td>  
    <td colspan="2">{$attendance[att].LOGIN|date_format:"%d-%m-%Y %I:%M %p"}</td>
    <td colspan="2">{$attendance[att].LOGOUT|date_format:"%d-%m-%Y %I:%M %p"}</td> 
  </tr>
{sectionelse}
  <tr><td colspan="1">No DATA</td></tr>
{/section}
</table>
</fieldset>
</div>

Thank you