Fix NoneType error in cart checkout

- Add null check for stripe_user_shop before accessing active_card property
- Prevents AttributeError: 'NoneType' object has no attribute 'active_card'
- Handle case where stripe_user_shop is None in cart checkout flow

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

make_post_sell/views/cart.py

@@ -455,7 +455,7 @@ def cart_checkout(request):
             request.session.flash(msg)
             return HTTPFound("/billing")
 
-        if stripe_user_shop.active_card is None:
+        if stripe_user_shop and stripe_user_shop.active_card is None:
             msg = ("Please make a payment method active.", "info")
             request.session.flash(msg)
             return HTTPFound("/billing")
@@ -465,7 +465,7 @@ def cart_checkout(request):
         return {
             "cart": cart,
             "products": cart.products,
-            "active_card": stripe_user_shop.active_card,
+            "active_card": stripe_user_shop.active_card if stripe_user_shop else None,
         }
 
     request.session.flash(msg)