fix(frontier): only allow gt 0 amount billing usage (#367)

Signed-off-by: Kush Sharma <thekushsharma@gmail.com>

raystack/frontier/v1beta1/models.proto

@@ -813,7 +813,7 @@ message Usage {
       "feature"
     ]
   }];
-  int64 amount = 6 [(validate.rules).int64 = {gte: 0}];
+  int64 amount = 6 [(validate.rules).int64 = {gt: 0}];
 
   // user_id is the user that triggered this usage
   string user_id = 7;