Fix output when hljs multi-line element contains an empty line

When a highlightjs multi-line element (for instance a multi-line comment)
contains an empty line, the resulting output numbered line was containing
a "{1}" string.
This fix handles the empty line special case.

Author: anlambert

dist/highlightjs-line-numbers.min.js

@@ -150,7 +150,8 @@
         var lines = getLines(element.innerHTML);
 
         for (var i = 0, result = ''; i < lines.length; i++) {
-            result += format('<span class="{0}">{1}</span>\n', [ className, lines[i] ]);
+          var lineText = lines[i].length > 0 ? lines[i] : ' ';
+          result += format('<span class="{0}">{1}</span>\n', [ className,  lineText ]);
         }
 
         element.innerHTML = result.trim();