Don't create empty attributes

Created by: Henni

This fixes #15359 (closed) by applying the changes proposed by @melissamanrique at https://github.com/twbs/bootstrap/pull/15360#issuecomment-67184355.

js/tooltip.js

@@ -342,7 +342,7 @@
 
   Tooltip.prototype.fixTitle = function () {
     var $e = this.$element
-    if ($e.attr('title') || typeof $e.attr('data-original-title') != 'string') {
+    if ($e.attr('title') && typeof $e.attr('data-original-title') != 'string') {
       $e.attr('data-original-title', $e.attr('title') || '').attr('title', '')
     }
   }