parse_subscription: Handle case where next_payment_date is the empty

string. This happens when a subscription is cancelled.
Signed-off-by: Birin Sanchez <birin.sanchez@puri.sm>

limitmonitor/common.py

@@ -42,10 +42,12 @@ def parse_subscription(json_entry):
                         % (quantity, id, ))
 
     # calculate next renewal date
-
-    next_renewal_naive = datetime.datetime.strptime(
-        json_entry["next_payment_date"], "%Y-%m-%dT%H:%M:%S")
-    next_renewal = timezone.make_aware(next_renewal_naive)
+    if json_entry["next_payment_date"] != "":
+        next_renewal_naive = datetime.datetime.strptime(
+            json_entry["next_payment_date"], "%Y-%m-%dT%H:%M:%S")
+        next_renewal = timezone.make_aware(next_renewal_naive)
+    else:
+        next_renewal = datetime.datetime(1900, 1, 1, 0, 0)
 
     # get account name
     username = get_username_from_woo_customer_id(json_entry["customer_id"])