Check get_url_parts return value in automatic redirect creation

Addresses #7999. get_url_parts _should_ always return a valid result since we only run it on sites that the page belongs to, but if it doesn't, we don't want it to error out (since this is only a background task).

wagtail/contrib/redirects/signal_handlers.py

@@ -95,7 +95,10 @@ def _page_urls_for_sites(page: Page, sites: Tuple[Site], cache_target: Page) ->
         if hasattr(cache_target, '_wagtail_cached_site_root_paths'):
             request._wagtail_cached_site_root_paths = cache_target._wagtail_cached_site_root_paths
 
-        site_id, root_url, page_path = page.get_url_parts(request)
+        url_parts = page.get_url_parts(request)
+        if url_parts is None:
+            continue
+        site_id, root_url, page_path = url_parts
 
         if page_path:
             for route_path in page.get_route_paths():