Fix PHP 8.4 deprecation: explicit nullable type for $context parameter in EntityConflictDiscoveryEvent

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.

In EntityConflictDiscoveryEvent::__construct(), the $context parameter is
declared as:

ParameterBag $context = NULL

This makes the type implicitly nullable, which is deprecated in PHP 8.4 and will
become a fatal error in a future version.

Steps to reproduce

1. Run Drupal with PHP 8.4.
2. Trigger any code path that instantiates
   Drupal\conflict\Event\EntityConflictDiscoveryEvent without passing a
   $context argument (i.e. relying on the NULL default).
3. Observe the PHP 8.4 deprecation notice:
   Implicitly marking parameter $context as nullable is deprecated, the
   explicit nullable type (?) must be used instead.

Proposed resolution

Add an explicit ? nullable marker to the $context parameter type hint in
EntityConflictDiscoveryEvent::__construct():

 // Before (PHP 8.4 deprecated):
 public function __construct(EntityInterface $local, EntityInterface $remote, EntityInterface $base, ParameterBag $context = NULL)

 // After (explicit nullable):
 public function __construct(EntityInterface $local, EntityInterface $remote, EntityInterface $base, ?ParameterBag $context = NULL)

The constructor body already handles a null value correctly via the null
coalescing operator ($this->context = $context ?? new ParameterBag()), so no
further changes are required.

Remaining tasks

• Apply the patch.
• Confirm no PHP 8.4 deprecation notices are emitted for this class.

User interface changes

None.

API changes

The signature of EntityConflictDiscoveryEvent::__construct() changes from an
implicitly nullable ParameterBag $context = NULL to an explicitly nullable
?ParameterBag $context = NULL. Callers that already pass NULL or omit the
argument are unaffected.

Data model changes

None.