improve method set_time_zone.

Author: Xu-Zhiqing

app/controllers/application_controller.rb

@@ -60,6 +60,6 @@ def #{s}!(redirect_url: nil)
 
   private
     def set_time_zone
-      Time.zone = current_user.time_zone if current_user and current_user.time_zone.present?
+      Time.zone = current_user.time_zone if current_user&.time_zone&.present?
     end
 end