DiscoveryCachedTrait::$definitions @var docblock should be array|null to match runtime semantics

Problem/Motivation

Drupal\Component\Plugin\Discovery\DiscoveryCachedTrait declares the cached-definitions property as:

/**
 * Cached definitions array.
 *
 * @var array
 */
protected $definitions;

But the runtime semantics allow NULL. Drupal\Core\Plugin\DefaultPluginManager::clearCachedDefinitions() explicitly does $this->definitions = NULL; to invalidate the local in-memory cache, after which getCachedDefinitions() relies on isset($this->definitions) returning FALSE to trigger a re-fetch from the cache backend.

The @var docblock should therefore declare array|null, not array. This is a pure docblock-correctness fix; runtime behavior is unchanged.

Why it matters

Downstream plugin manager classes that extend DefaultPluginManager and override clearCachedDefinitions() (e.g. to walk per-language cache bins for language-aware caching) trip PHPStan's assign.propertyType rule when they do the same $this->definitions = NULL; that core itself does. PHPStan reads the @var docblock as authoritative and flags the assignment as type-violating.

Recent contrib MRs running into this:

• d.o 3589007 ? core ActionManager language-aware cache.
• d.o 3589765 ? VBO ViewsBulkOperationsActionManager language-aware cache.

Proposed resolution

Update the @var docblock in DiscoveryCachedTrait from array to array|null.