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$("td:empty").text("Notu girilmemiş").css('background', 'red');

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<table border="1">
<tr>
   <td>No</td><td>Ad Soyad</td><td>Sınıf</td><td>Sınav</td>
</tr><tr>
    <td>1523</td><td>Alp Sönmez</td><td>5A</td><td></td>
</tr><tr>
    <td>669</td><td>Aysun Demirci</td><td>5B</td><td>80</td>
</tr><tr>
    <td>7189</td><td>Mahmut Toprak</td><td>3D</td><td></td>
</tr>
</table>