This is my table structure, I want to find out the index of the tr which first td is empty, in this case, the index should be 1, how to pick it out?

<table width="100%" border="1" cellspacing="0" cellpadding="0">

  <tr>

    <td>1</td>

    <td>1</td>

    <td>1</td>

    <td></td>

  </tr>

  <tr>

    <td></td>

    <td>1</td>

    <td>1</td>

    <td>1</td>

  </tr>

  <tr>

    <td>2</td>

    <td>1</td>

    <td>1</td>

    <td>1</td>

  </tr>

  <tr>

    <td></td>

    <td>1</td>

    <td>1</td>

    <td>1</td>

  </tr>

</table>