Handle None when all stripe plans are empty

stripe_utils.py

@@ -267,10 +267,11 @@ class StripeUtils(object):
     def _get_all_stripe_plans(self):
         all_stripe_plans = client.get("/v1/stripe_plans")
         all_stripe_plans_set = set()
-        all_stripe_plans_obj = json.loads(all_stripe_plans.value)
-        if (all_stripe_plans_obj and
-            len(all_stripe_plans_obj['plans']) > 0):
-            all_stripe_plans_set = set(all_stripe_plans_obj["plans"])
+        if all_stripe_plans:
+            all_stripe_plans_obj = json.loads(all_stripe_plans.value)
+            if (all_stripe_plans_obj and
+                len(all_stripe_plans_obj['plans']) > 0):
+                all_stripe_plans_set = set(all_stripe_plans_obj["plans"])
         return all_stripe_plans_set
 
     def _save_all_stripe_plans(self, stripe_plans):