Multiple items in cart not working in case of existing=empty

Problem/Motivation

Adding multiple items to the cart will not work when existing=empty because the for loop empty the cart on each loop which cause only the last items to be added to the cart

Steps to reproduce

try to add multiple items by link and set the existing to empty

Proposed resolution

fix the for loop

Remaining tasks

Patch

User interface changes

none

API changes

none

Data model changes

none