Check for empty <tr> in pbgc scraper before attempting to set variable b...

Some of the markup on the PBGC site isn't valid (using tr with no slash as a closing tag), which is causing trouble:

```
			<tr>
				<td  style='border:1px solid #86b2d8;'><a href='pdfs/LTR-2014-09.pdf' target='_blank'>OIG Response to Rep. Turner Request - Delphi Salaried Pension Plan Delays</a></td>
				<td style='border:1px solid #86b2d8;' align='center'>LTR 2014-13</td>
				<td style='border:1px solid #86b2d8;' align='center'>09/02/2014</td>
			
			<tr>
```

So this commit just does a check to make sure that the tr has a nested td before doing its thing.