[8.x] Fix timezone option in schedule:list command

[royvanv]

Currently php artisan schedule:list fails with the following error:

 DateTime::setTimezone(): Argument #1 ($timezone) must be of type DateTimeZone, string given

This PR converts the timezone value from a string to an instance of DateTimeZone. It also sets default value with null coalescing check, because the options method does not have a 2nd parameter.