corporate: Fix 'get_next_billing_cycle' for scheduled fixed-price plan.

No renewal ledger entry exists for a scheduled fixed-price plan.

Earlier, when a customer with scheduled fixed-price plan
visited the billing page, the 'get_next_billing_cycle' function
raised an assertion error as no renewal ledger entry exists.

We don't need the ledger entry to determine the next billing
cycle in this case, so we move the code block which determines
the last_renewal time within the else conditional i.e. where it
is needed.

The scheduled plan has NEVER_STARTED status and the
next_invoice_date set to end date of the current plan.
We use this info to determine the next billing cycle.

corporate/lib/stripe.py

@@ -1994,15 +1994,10 @@ class BillingSession(ABC):
             )
 
     def get_next_billing_cycle(self, plan: CustomerPlan) -> datetime:
-        last_ledger_renewal = (
-            LicenseLedger.objects.filter(plan=plan, is_renewal=True).order_by("-id").first()
-        )
-        assert last_ledger_renewal is not None
-        last_renewal = last_ledger_renewal.event_time
-
         if plan.status in (
             CustomerPlan.FREE_TRIAL,
             CustomerPlan.DOWNGRADE_AT_END_OF_FREE_TRIAL,
+            CustomerPlan.NEVER_STARTED,
         ):
             assert plan.next_invoice_date is not None
             next_billing_cycle = plan.next_invoice_date
@@ -2010,6 +2005,11 @@ class BillingSession(ABC):
             assert plan.end_date is not None
             next_billing_cycle = plan.end_date
         else:
+            last_ledger_renewal = (
+                LicenseLedger.objects.filter(plan=plan, is_renewal=True).order_by("-id").first()
+            )
+            assert last_ledger_renewal is not None
+            last_renewal = last_ledger_renewal.event_time
             next_billing_cycle = start_of_next_billing_cycle(plan, last_renewal)
 
         if plan.end_date is not None: