source-stripe-native: make created optional for Accounts

Sometimes, `account.updated` events do not have `created` in the document, causing failures when we validate the document against its model. Making `created` optional will avoid these failures and reflect the API's actual response.

source-stripe-native/source_stripe_native/models.py

@@ -151,6 +151,8 @@ class Accounts(BaseStripeObjectWithEvents):
         "account.updated": "u",
     }
 
+    # Accounts docs returned in account.updated events may not have a created field.
+    created: int | None = None
 
 # Could not verify Persons events are generated in test mode, but suspect
 # they are generated in Stripe's live mode.