Fix bug where links without urls failed

Author: wooorm

lib/util/parse-link.js

@@ -37,7 +37,7 @@ var CC_HASH = '#'.charCodeAt(0);
  */
 function parse(node) {
   var link = {};
-  var url = node.url || /* istanbul ignore next - remark@<4.0.0 */ node.href;
+  var url = node.url || node.href || '';
   var start;
   var end;
   var page;

test/fixtures/links/input.md

@@ -11,6 +11,8 @@ pretty.
 
 ## Non-references
 
+An empty link: [this and that]().
+
 A link to GH:
 <https://github.com>
 

test/fixtures/links/output.md

@@ -11,6 +11,8 @@ pretty.
 
 ## Non-references
 
+An empty link: [this and that](<>).
+
 A link to GH:
 <https://github.com>