PHP 8.4: fix implicit nullable deprecations

Problem/Motivation

PHP 8.4 introduces a deprecation for implicitly nullable parameters: https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullabl...

Steps to reproduce

Install module on Drupal 10.5 with PHP 8.4. Log is filled with deprecation notices such as:
Deprecated: Drupal\easychart\Form\OptionsForm::buildForm(): Implicitly marking parameter $request as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/easychart/src/Form/OptionsForm.php on line 53

Proposed resolution

Adjust the nullable parameters to declare them as nullable.