Deprecation notice: null used as array offset in redirect_status_code_options()

Problem/Motivation

On PHP 8.5+, visiting the redirect settings page (/admin/config/search/redirect/settings) triggers a deprecation notice:

Deprecated function: Using null as an array offset is deprecated, use an empty string instead in redirect_status_code_options() (line 205 of redirect.module)

The function signature has a default of $code = NULL, and line 205 uses $codes[$code] directly in the null coalescing expression. When called without arguments (as RedirectSettingsForm::buildForm() does at line 70), $code is NULL, and PHP 8.5+ deprecates using null as an array key.

Steps to reproduce

1. Use PHP 8.5 or later with deprecation notices visible.
2. Go to Administration > Configuration > Search and metadata > URL redirects > Settings.
3. Observe the deprecation notice.

Proposed resolution

Check for NULL explicitly before indexing:

- return $codes[$code] ?? $codes;
+ return $code !== NULL ? $codes[$code] : $codes;