[PHP 8.5] ContentTranslationManageAccessCheck passes NULL to LanguageManager::getLanguage()

Problem/Motivation

When visiting a content translation add page (e.g. /node/1/translations/add/en/af), PathBasedBreadcrumbBuilder checks access for each parent path segment. Because the entity.{entity_type}.content_translation_add route defines both {source} and {target} as optional parameters (defaulting to NULL), the breadcrumb builder also triggers access checks for partial paths such as /node/1/translations/add/en (source=en, target=NULL) and /node/1/translations/add (source=NULL, target=NULL).

ContentTranslationManageAccessCheck::access() passes these NULL values directly to LanguageManager::getLanguage(), which performs an array lookup $languages[$langcode]. Using NULL as an array offset is deprecated in PHP 8.5, producing:

Deprecated function: Using null as an array offset is deprecated, use an empty string instead in Drupal\Core\Language\LanguageManager->getLanguage()

The same method already guards the $language parameter correctly (lines 73–76), but the $source and $target parameters on lines 104–105 were missing the equivalent null check.

Steps to reproduce

1. Enable content_translation and configure at least two languages.
2. Create a translatable node.
3. Log in as a non-superadmin user with translation permissions.
4. Visit /node/{nid}/translations/add/{source}/{target}.
5. Observe the PHP 8.5 deprecation notice in the logs

Proposed resolution

- Guard $source and $target before passing them to getLanguage(), consistent with the existing pattern used in other places.
- Add a unit/kernel test showcasing the missing coverage.

Remaining tasks

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User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

None