Losing cart when user is logging

[korycins]

Steps to reproduce the problem

1. Add to cart any product
2. Go to the checkout
3. Sign in
4. in some cases checkout cart is lost

System information Operating system: OSX Browser: Chrome Storefront Branch: master

[tommytobi]

This is appears to be caused by an ambiguity in the graphql schema description and result, for mutation checkoutcreate. when a user has a previously abandoned checkout, instead of creating a new checkout, an old checkout is returned instead. If the old checkout has no items in the cart you get the result above.

issue created in saleor-api: CheckoutCreate mutation returns open checkout when creating new one #6185

FYI: commenting out lines 301-> 306 in saleor/graphql/checkout/mutations.py quick fixes this problem (and possibly creates others)