fix: update default parameter from null to empty array in getSettings…

[NadsonKT]

This pull request includes changes to the NovaSettings functionality to ensure that the getSettings method handles default values more consistently. The most important changes include modifying the method signatures in several files to use empty arrays as default parameters instead of null.

Consistency improvements to getSettings method:

• src/NovaSettings.php: Changed the getSettings method to use an empty array as the default value for settingKeys instead of null.
• src/NovaSettingsStore.php: Updated the getSettings method to use an empty array as the default value for settingKeys instead of null.
• src/helpers.php: Modified the nova_get_settings function to use an empty array as the default value for settingKeys instead of null.