Update JQuery visibility check on sticky element

Following a PR review[1], this commit changes the test on this.$sticky, in
order to verify if the object actually contains any elements.

Further details in the PR[2].

[1]
#368 (comment)

[2]
#368

lib/assets/javascripts/_modules/table-of-contents.js

@@ -10,7 +10,7 @@
     this.isMonitoring = false
   }
   StickyOverlapMonitors.prototype.run = function () {
-    var stickyIsVisible = this.$sticky && this.$sticky.is(':visible')
+    var stickyIsVisible = this.$sticky.length > 0 && this.$sticky.is(':visible')
     if (stickyIsVisible && !this.isMonitoring) {
       document.addEventListener('focus', this.onFocus, true)
       this.isMonitoring = true