Do not skip empty table rows. Fixes domchristie#276.

[michbart]

Current implementation skips empty table rows, but it shouldn't as the specification says, that there can be empty row. This PR fixes #276, I didn't add TABLE intentionally as it was in #277.

Example 1:

<table>
  <thead>
    <tr>
      <th>Header</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td></td>
    </tr>
    <tr>
      <td>Cell</td>
    </tr>
  </tbody>
</table>

Should be

| Header |
| --- |
|  |
| Cell |

Example 2:

<table>
  <thead>
    <tr>
      <th></th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Cell</td>
    </tr>
  </tbody>
</table>

Should be

|  |
| --- |
| Cell |

[domchristie]

Thanks for this. I have used this idea as part of #326