fix: highlight line issue for empty lines

Author: ulivz

lib/markdown/highlightLines.js

@@ -39,7 +39,7 @@ module.exports = md => {
       })
       if (inRange) {
         return {
-          code: `<span class="highlighted-line">${split}</span>`,
+          code: `<span class="highlighted-line">${split || '\n'}</span>`,
           highlighted: true
         }
       }