Fix LocalePathFunctionalTest and system_custom_theme()

+++ b/core/lib/Drupal/Core/EventSubscriber/PathListenerBase.php
@@ -16,7 +16,8 @@ use Symfony\Component\HttpFoundation\Request;
-    return $request->attributes->get('system_path') ?: trim($request->getPathInfo(), '/');
+    $path = $request->attributes->get('system_path');
+    return isset($path) ? $path : trim($request->getPathInfo(), '/');

This part fixes LocalePathFunctionalTest.

+++ b/core/modules/system/system.module
@@ -2021,7 +2021,7 @@ function system_add_module_assets() {
-    $path = ltrim($request->getPathInfo(), '/');
+    $path = $request->attributes->get('system_path');

I don't know if this fixes a test, or if we're missing test coverage for 'xx/admin' returning the admin theme if xx is a language prefix. Similarly for aliases (e.g., if 'foo' is an alias for 'admin', then 'foo' should return the admin theme).

Sorry. Just noticed by looking at other issues in this project, that "bug report" is the correct category.

Lol. And sorry for the xposts :)

why the shortened ternary did not work here

Cause if you have a path like 'xx' representing the home page in language xx, then after langcode stripping, system_path is empty string, which evaluates as FALSE. In this case, we want the next listener (home page resolver) getting back the empty string, not falling back to the request path.