tier1 always be None in GET tier config request for level 2

Without having null allowable SDk will throw the following error  
TypeError: Invalid structure for initialization of `TierConfigRequest`. 
Error: {0: {'tiers': {'tier1': ['Field may not be null.']}}, 1: {'tiers': {'tier1': ['Field may not be null.']}}}.

Because in t2 config request t1 account always be none as per api result from connect platform

Author: shalder07

connect/models/schemas.py

@@ -501,7 +501,7 @@ def make_object(self, data):
 
 class TierAccountsSchema(Schema):
     customer = fields.Nested(TierAccountSchema)
-    tier1 = fields.Nested(TierAccountSchema)
+    tier1 = fields.Nested(TierAccountSchema, allow_none=True)
     tier2 = fields.Nested(TierAccountSchema, allow_none=True)
 
     @post_load