Ensure default footer actions' media is included

It was popped from the menu_items, so it's not in that list when we loop
through to collect the media files.

And add an early return if the default item is undefined (e.g. if a
construct hook caused the list to be empty), so it doesn't crash when it
tries to render to action menu.

wagtail/admin/action_menu.py

@@ -317,6 +317,9 @@ class PageActionMenu:
             self.default_item = None
 
     def render_html(self):
+        if not self.default_item:
+            return ""
+
         rendered_menu_items = [
             menu_item.render_html(self.context) for menu_item in self.menu_items
         ]
@@ -335,7 +338,7 @@ class PageActionMenu:
 
     @cached_property
     def media(self):
-        media = Media()
+        media = self.default_item.media if self.default_item else Media()
         for item in self.menu_items:
             media += item.media
         return media

wagtail/snippets/action_menu.py

@@ -368,7 +368,7 @@ class SnippetActionMenu:
 
     @cached_property
     def media(self):
-        media = Media()
+        media = self.default_item.media if self.default_item else Media()
         for item in self.menu_items:
             media += item.media
         return media