Fixes #7730: Better hiding of empty titles in popovers

If the popover's title contains no text, hide it via JS. Previously was
done via CSS's `:empty` property, but that doesn't work in IE8, so JS
it is.

/cc @fat becaue I probably did this wrong <3

docs/assets/js/bootstrap.js

@@ -1420,6 +1420,8 @@
     $tip.find('.popover-content')[this.options.html ? 'html' : 'text'](content)
 
     $tip.removeClass('fade top bottom left right in')
+
+    $tip.find('.popover-title:empty').hide()
   }
 
   Popover.prototype.hasContent = function () {