Fix Session store not set on request

[aroutinr]

This change fixes the error in vendor/laravel/framework/src/Illuminate/Http/Request.php:560 when the request does not have a session. This happens to me when I use the package with Laravel Passport.

The previous evaluation was incorrect since if the request does not have a session but the configuration variable is not "off" it still evaluates to false and the execution of the code continues.

With the change I made in this PR the execution of the code will continue only when the request has a session and config('timezone.flash') is not "off".

Setting the config('timezone.flash') to "off" also fixes the problem as mentioned here but i think is not the right solution for the problem: https://github.com/jamesmills/laravel-timezone/issues/59#issuecomment-914325684