fixed if/else chain

mapping_provider/backends/rmq/exchange.py

@@ -196,9 +196,9 @@ class RabbitMQClient:
         self.exchange_type = exchange_type
 
         self.routing_keys: Sequence[str] = []  # for mypy
-        if routing_keys is not None:
+        if routing_keys is None:
             self.routing_keys = []
-        if isinstance(routing_keys, str):
+        elif isinstance(routing_keys, str):
             self.routing_keys = [routing_keys]
         elif isinstance(routing_keys, Sequence):
             self.routing_keys = routing_keys