Bug: Adding a smart menu divider does not work for user menu submenus

[abias]

Steps to reproduce:

• Create a smart menu which is set up to be displayed in the main navigation and the user menu
• Add the following items to this smart menu:
  • A static menu item with an arbitrary content
  • A heading menu item with the title ### which will result in the menu item being displayed as a divider
  • Another static menu item with an arbitrary content
• Have a look at the resulting menus

Expected result:

• In the main navigation, there is a divider shown before the first and the second static menu item
• In the user menu, there is a divider shown before the first and the second static menu item

Actual result:

• In the main navigation, there is a divider shown before the first and the second static menu item
• In the user menu, there isn't any divider shown before the first and the second static menu item

Screenshots:

[abias]

Note: As soon as this bug is solved, the CSS solution in https://github.com/moodle-an-hochschulen/moodle-theme_boost_union/pull/538/files#diff-9cef13c9077e3d8bf0379912a31de527ec48a5f7c67dc9e6d7f94b1bafd23cc7R318-R330 should be rebuilt.