getUrl() fatals for menu_link_content entities generated from MenuLinkDefault links

Hi,

Thanks as always to all helping to maintain the module (:

Disclaimer: I don't know it well and got a little confused trying to follow through at times, so I could easily be wrong...

Problem/Motivation

1. Install the standard profile: you should have a Home link by default on the Main navigation menu.
2. Add a field to the main menu bundle. (I don't know why this is necessary.)
3. Add a (menu_link_content) link to the main menu.
4. In hook_preprocess_menu_link_content() run $variables['menu_link_content']->getUrlObject();

The following error is generated when preprocessing the Home link:

The website encountered an unexpected error. Please try again later.
Error: Call to a member function getUrl() on null in Drupal\menu_link_content\Entity\MenuLinkContent->getUrlObject() (line 88 of core/modules/menu_link_content/src/Entity/MenuLinkContent.php).

If I understand correctly, currently in \Drupal\menu_item_extras\Service\MenuLinkTreeHandler::getMenuLinkItemEntity() it returns a menu_link_content entity regardless of the plugin type. I think it's connected with this PR and issue GH#98. In particular it looks like...

      $menu_item = $this->entityTypeManager
        ->getStorage('menu_link_content')
        ->create($link->getPluginDefinition());

...doesn't pass a value for the link base field when used with MenuLinkDefault.

Proposed resolution

I was wondering if it really makes sense to create a menu link content entity for other types of link (fwiw it came as a surprise to me when I found out), but I don't know the rationale behind that decision. So I was wondering if it might make sense to either

1. not create a menu_link_content entity for all links;
2. use \Drupal\Core\Menu\MenuLinkInterface to initialise the content entity from the menu plugin.

Please let me know if I can provide any more debug info, and I'd be happy to help out with a solution.

Thanks!