When referencing links with empty optional parameters, the converter fails

```
What steps will reproduce the problem?
1. write
[14]:http://blog.thecanadaline.com/wp-content/uploads/2012/03/IMG_62441-225x300.
jpg ()\n\n
2. try to convert
3.

What is the expected output? What do you see instead?
we are expecting to have the link correctly coded inside the page, instead we
have a broken link and the unconverted string appears at the end of the page as
text:
[14]:http://blog.thecanadaline.com/wp-content/uploads/2012/03/IMG_62441-225x300.
jpg ()\n\n

We should be able to furnish empty parameters in brackets without breaking the
behaviour of the app...

What version of the product are you using? On what operating system?

Please provide any additional information below.
Removing the empty braces fixes the markdown.
```

Original issue reported on code.google.com by `dmar...@allscenes.com` on 13 Sep 2012 at 11:32