Fix deprecations in PHP 8.4: 'Implicitly marking parameter as nullable is deprecated'

Problem/Motivation

PHP Deprecated: Drupal\forms_steps\Entity\FormsSteps::getSteps(): Implicitly marking parameter $step_ids as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/Entity/FormsSteps.php on line 352

PHP Deprecated: Drupal\forms_steps\Entity\FormsSteps::getProgressSteps(): Implicitly marking parameter $progress_step_ids as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/Entity/FormsSteps.php on line 365

PHP Deprecated: Drupal\forms_steps\Entity\FormsSteps::getFirstStep(): Implicitly marking parameter $steps as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/Entity/FormsSteps.php on line 405

PHP Deprecated: Drupal\forms_steps\Entity\FormsSteps::getLastStep(): Implicitly marking parameter $steps as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/Entity/FormsSteps.php on line 416

PHP Deprecated: Drupal\forms_steps\FormsStepsInterface::getSteps(): Implicitly marking parameter $step_ids as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/FormsStepsInterface.php on line 113

PHP Deprecated: Drupal\forms_steps\FormsStepsInterface::getProgressSteps(): Implicitly marking parameter $progress_step_ids as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/FormsStepsInterface.php on line 129

PHP Deprecated: Drupal\forms_steps\FormsStepsInterface::getLastStep(): Implicitly marking parameter $steps as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/FormsStepsInterface.php on line 140

PHP Deprecated: Drupal\forms_steps\FormsStepsInterface::getFirstStep(): Implicitly marking parameter $steps as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/web/modules/contrib/forms_steps/src/FormsStepsInterface.php on line 151

Steps to reproduce

Upgrade PHP to 8.4

Proposed resolution

Remaining tasks

User interface changes

None.

API changes

No, only making what is allowed explicit.

Data model changes

None.