Further examples for inline links (empty link text)

[jsteuer]

The specification says

A link text consists of a sequence of zero or more inline elements enclosed by square brackets ([ and ])

Please add examples that covers some edge cases:

```````````````````````````````` example
[](./target.md)
.
<p><a href="./target.md"></a></p>
````````````````````````````````

```````````````````````````````` example
[]()
.
<p><a href=""></a></p>
````````````````````````````````

Note:

• This follows the spec and is the current behaviour of the online demo (Quick Link)
• Empty link texts are useful: some application can lookup and insert the heading of referenced documents for example.
• Since some parsers are tested against the examples, it would be great to cover this behaviour.

If desired, I can create a pull request.

Thank you!