Hotfix don't assume currency is present in event checkout.session.completed

[chrisjsimpson]

Is your feature request related to a problem? Please describe.

checkout.session.completed event processing assumes currency is always present , however this may not always be the case since currency is a nullable property.

subscribie/subscribie/blueprints/checkout/__init__.py

Line 862 in b27ffc0

currency = session["currency"].upper()

Describe the solution you'd like
Don't assume currency is present.

Describe alternatives you've considered

Additional context