dbus channel: make 'close' message idempotent

It's theoretically possible that the user might send multiple 'close'
messages to a D-Bus channel, which would have a chance to be delivered
because the channel doesn't close immediately (but rather waits for
outstanding requests to complete).

This is a pretty theoretical bug but it's going to get worse when we
modify how the router shutdown works in the next commit.

src/cockpit/channels/dbus.py

@@ -511,5 +511,5 @@ def do_data(self, data):
     def do_close(self):
         for slot in self.matches:
             slot.cancel()
-        self.matches = None  # error out
+        self.matches = []
         self.close()