[FIX] web_editor: hide header in customize dropdown when it has no content

In some snippets, the parent hierarchy has many elements. So, when
selecting a deep child of one of those, the customize dropdown had
many dropdown headers showing the parent hierarchy but without any
option for many of them.

Commit de0d9fd666 already fixed the case
where the dropdown only contains dropdown headers. This commit is an
adaptation to hide these headers one by one.

addons/web_editor/static/src/js/snippets.editor.js

@@ -1122,22 +1122,29 @@ data.Editor = Class.extend({
             if ($el.data("editor") !== this) {
                 styles = _.filter(styles, function (option) { return !option.preventChildPropagation; });
             }
+
+            var count = 0;
             _.each(_.sortBy(styles, "__order").reverse(), function (style) {
-                do_action(style, $el);
+                if (do_action(style, $el)) {
+                    count++;
+                }
             });
+            $el.toggleClass("hidden", count === 0);
         }).bind(this));
 
         // Activate the overlay
-        $style_button.toggleClass("hidden", $ul.children(":not(.dropdown-header):not(.divider):not(.hidden)").length === 0);
+        $style_button.toggleClass("hidden", $ul.children(":not(.divider):not(.hidden)").length === 0);
         this.$overlay.toggleClass("oe_active", !!focus);
 
         function _do_action_focus(style, $dest) {
             style.$el.insertAfter($dest);
             style.on_focus();
+            return (style.$el.length > 0);
         }
         function _do_action_blur(style, $dest) {
             style.$el.detach();
             style.on_blur();
+            return false;
         }
     },
 });