bat 8.x-10.3-rc1

Fix Issue #3554606: Make user_settings parameter optional in bat_fullcalendar_configure

- Change function signature to use default parameter value
- Add null coalescing operator for safe array access in foreach loop
- Maintain backward compatibility with existing implementations
- Follow Drupal coding standards for optional parameters

This allows the function to be called without parameters while preserving existing functionality for modules that pass user_settings.