add empty comment test. Closes #25

Author: tj

test/cases/comment.url.css

@@ -1,4 +1,5 @@
 /* http://foo.com/bar/baz.html */
+/**/
 
 foo { /*/*/
   /* something */

test/cases/comment.url.json

@@ -4,6 +4,9 @@
       {
         "comment": " http://foo.com/bar/baz.html "
       },
+      {
+        "comment": ""
+      },
       {
         "selectors": [
           "foo"
@@ -17,4 +20,4 @@
       }
     ]
   }
-}
+}