Allow inline links with empty link text

[jsteuer]

1. What version of goldmark are you using? : v1.1.22

2. What version of Go are you using? : 1.13.7

3. What operating system and processor architecture are you using? : linux/amd64

4. What did you do? :

   I need to process links with emtpy link text - for example [](./target.md). My usecase is to write an extension which find the heading of the referenced document and set it as the link text, if its empty.

   package main
   
   import (
       "bytes"
   
       "github.com/yuin/goldmark"
   )
   
   func main() {
       var source = []byte(`[](./target.md)`)
       var buf bytes.Buffer
       if err := goldmark.Convert(source, &buf); err != nil {
           panic(err)
       }
       print(buf.String())
   }

5. What did you expect to see? :

   <p><a href="./target.md"></a></p>

   Note: I need the corresponding Node in the AST

6. What did you see instead? :

   <p>[](./target.md)</p>

7. Did you confirm your output is different from CommonMark online demo or other official renderer correspond with an extension?:

   Yes, the CommonMark specification (https://spec.commonmark.org/0.29/#link-text) says:

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

   But there is no example for such a edge case. Issue is created: https://github.com/commonmark/commonmark-spec/issues/636

   Input (the last one is really useless but conform with the specification)

   [My Target](./target.md)
   [](./target.md)
   []()

   CommonMark online demo ([Quick Link](https://spec.commonmark.org/dingus/?text=%5BMy%20Target%5D(.%2Ftarget.md)%0A%5B%5D(.%2Ftarget.md)%0A%5B%5D()%0A)) output

   <p><a href="./target.md">My Target</a>
   <a href="./target.md"></a>
   <a href=""></a></p>

   Goldmark output

   <p><a href="./target.md">My Target</a>
   [](./target.md)
   []()</p>