PHP 8.4 deprecations: implicitly nullable FilterInterface parameters in ShortcodeService

### Problem

`ShortcodeService` triggers PHP deprecation notices on PHP 8.4 because several `FilterInterface` parameters default to `NULL` without being explicitly nullable.

### Steps to Reproduce

1. Install Shortcode 2.0.3 on Drupal 11.
2. Run the site with PHP 8.4 or newer and deprecation notices enabled.
3. Process text through a text format/filter that uses Shortcode.

### Actual Result

The following deprecation notices are emitted:

Deprecated: Drupal\shortcode\ShortcodeService::getShortcodePlugins(): Implicitly marking parameter $filter as nullable is deprecated, the explicit nullable type must be used instead in modules/contrib/shortcode/src/ShortcodeService.php on line 89

Deprecated: Drupal\shortcode\ShortcodeService::process(): Implicitly marking parameter $filter as nullable is deprecated, the explicit nullable type must be used instead in modules/contrib/shortcode/src/ShortcodeService.php on line 215

Deprecated: Drupal\shortcode\ShortcodeService::postprocessText(): Implicitly marking parameter $filter as nullable is deprecated, the explicit nullable type must be used instead in modules/contrib/shortcode/src/ShortcodeService.php on line 360

### Expected Result

No PHP deprecation notices should be emitted on PHP 8.4.

### Proposed Fix

Change the affected method signatures to use explicit nullable types:

public function getShortcodePlugins(?FilterInterface $filter = NULL, bool $reset = FALSE): array

public function process($text, string $langcode = Language::LANGCODE_NOT_SPECIFIED, ?FilterInterface $filter = NULL): string

public function postprocessText($text, string $langcode, ?FilterInterface $filter = NULL): string

### Notes

This is a PHP 8.4 compatibility issue. The change is type-compatible with the existing default `NULL` behavior.