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

less/popovers.less

@@ -40,10 +40,6 @@
   background-color: @popover-title-bg;
   border-bottom: 1px solid darken(@popover-title-bg, 5%);
   border-radius: 5px 5px 0 0;
-
-  &:empty {
-    display: none;
-  }
 }
 
 .popover-content {