Go back with back button shop cart is empty

Pages are cached by your browser. When you click on the back button of the browser, it displays the page without reloading it which can lead to such problems.
There is nothing you can do to avoid that.

But it could also be that you have some cache activated on your website causing that.
You should :
Disable the cache in Joomla configuration.
Site -> global configuration -> system -> "Cache settings".
Disable, if it's enabled, a joomla cache plugin.
Check your template' options for a potential cache option and deactivate it.