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/css/bootstrap.css

@@ -4376,10 +4376,6 @@ button.close {
   border-radius: 5px 5px 0 0;
 }
 
-.popover-title:empty {
-  display: none;
-}
-
 .popover-content {
   padding: 9px 14px;
 }

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 () {

js/popover.js

@@ -55,6 +55,8 @@
     $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 () {

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 {