bugfix for routing keys introduced in mypy refactoring

mapping_provider/backends/rmq/queue.py

@@ -74,7 +74,7 @@ def setup_channel(
     queue_declare: bool = True,
     exclusive: bool = False,
     single_active_consumer: bool = False,
-    routing_keys: str | Sequence[str | None] | None = None,
+    routing_keys: Sequence[str] = [],
     prefetch_count: int | None = None,
     force_quorum_queue: bool = False,
 ) -> tuple[BlockingChannel, str | None]:
@@ -142,8 +142,11 @@ def setup_channel(
 
     assert queue_name, "queue name must not be empty here"
  
-    if routing_keys is None or isinstance(routing_keys, str):
-        routing_keys = [routing_keys]
+    if not routing_keys:
+        # in case no routing keys are provided (as for fanout exchanges),
+        # ensure the queue is still bound to the exchange
+        routing_keys = [None]
+
     for rk in routing_keys:
         channel.queue_bind(exchange=exchange_name, queue=queue_name, routing_key=rk)