There is no such restriction in Odoo for not showing the product if the quantity is zero. The idea behind the POS is to serve the customer on time, irrespective of quantity. 

Consider a case where you have the item physically and the customer takes it to the counter for billing. But in the system that item was not entered (maybe PO was missed or issue in the stock count). So, if the shop did not provide that product, it would be a bad experience for both the customer and the shop.

But you can use some 3rd party modules available in the app store.

https://apps.odoo.com/apps/modules/14.0/pw_pos_restrict_zero_qty/

https://apps.odoo.com/apps/modules/15.0/pos_not_allow_zero_quantity/

Note:- These are not tested from my end. Just saw it in the store.