fix media comments for discussions with tons of videos

if you called $().mediaCommentThumbnail() with an
empty set it would break it so it won't make mediaCommentThumbails
any more for anything else.

Change-Id: I1a8868439563df7655c07d1aedfada37ad4db857
fixes: #8874
Reviewed-on: https://gerrit.instructure.com/11522
Reviewed-by: Zach Wily <zach@instructure.com>
Tested-by: Zach Wily <zach@instructure.com>

Author: ryankshaw

public/javascripts/media_comments.js

@@ -146,7 +146,6 @@ define([
     var thumbnailsQueued = [];
     var thumbnailing = false;
     var nextThumbnail = function() {
-      if (!thumbnailsQueued.length) return; // shortcut if there's no work to do
       thumbnailing = true;
       var iterations = Math.min(thumbnailsQueued.length, 30),
           thumbnail;