Fix explicit nullable type PHP 8.4

[Eduardo-Morales-Alberti]

PHP 8.4 deprecation: implicit nullable parameter types

Problem/Motivation

PHP 8.4 deprecates implicitly nullable parameter types. When a typed parameter
has a default value of NULL without an explicit ? prefix on the type hint,
PHP 8.4 emits a deprecation warning like:

Implicitly marking parameter $foo as nullable is deprecated, the explicit
nullable type (?) must be used instead.

The following signatures across behat-contexts are affected:

src/Behat/Context/EntityContext.php (L116)
protected function visitEntityPath(string $entity_type, $entity, string $subpath = NULL, $language = NULL)

src/Behat/Cores/CoreInterface.php (L54, L401)
public function staticEntityCacheClear($entity_type_id, array $ids = NULL);
public function buildPath(string $path, string $langcode = NULL);

src/Behat/Cores/Drupal7.php (L41, L391)
public function staticEntityCacheClear($entity_type_id, array $ids = NULL)
public function buildPath(string $path, string $langcode = NULL)

src/Behat/Cores/Drupal8.php (L73, L586)
public function staticEntityCacheClear($entity_type_id, array $ids = NULL)
public function buildPath(string $path, string $langcode = NULL)

Steps to reproduce

1. Run a Behat test suite with PHP 8.4.
2. Trigger any code path that invokes one of the affected methods without
   passing the nullable argument (i.e. relying on the NULL default).
3. Observe PHP 8.4 deprecation notices for each affected signature.

Proposed resolution

Add an explicit ? nullable marker to each affected typed parameter.

// EntityContext.php — before:
protected function visitEntityPath(string $entity_type, $entity, string $subpath = NULL, $language = NULL)
// after:
protected function visitEntityPath(string $entity_type, $entity, ?string $subpath = NULL, $language = NULL)

// CoreInterface.php + Drupal7.php + Drupal8.php — before:
public function staticEntityCacheClear($entity_type_id, array $ids = NULL);
public function buildPath(string $path, string $langcode = NULL);
// after:
public function staticEntityCacheClear($entity_type_id, ?array $ids = NULL);
public function buildPath(string $path, ?string $langcode = NULL);

The implementations and callers are unaffected: any caller already passing
NULL or omitting the argument will continue to work identically.

Remaining tasks

• Apply the patch.
• Confirm no PHP 8.4 deprecation notices are emitted for the affected methods.

User interface changes

None.

API changes

The signatures of the following methods change from implicitly nullable to
explicitly nullable typed parameters. Callers that already pass NULL or omit
the argument are unaffected:

• EntityContext::visitEntityPath() — string $subpath
• CoreInterface::staticEntityCacheClear() / Drupal7 / Drupal8 — array $ids
• CoreInterface::buildPath() / Drupal7 / Drupal8 — string $langcode