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

查看 pagedown 程式庫中的 markdown 轉換器，可能位於類似 Markdown.Converter.js 的檔案中。找出帶有選用參數（例如空括號）的連結參照在哪裡進行剖析。使用提供的錯誤範例進行測試，以查看剖析在哪裡失敗。檢查是否已有針對連結參照的測試，以了解預期行為。

由索引模型根據 Issue 內容生成。