fix: Missing nil check for optional reply_tx

A previous refactor that made the "Unique Key" constructor fallible also added
sending the error condition back over the optional reply tx channel argument,
which is allowed to be nil.

The nil check over this argument was omitted, though, which can lead to crashes
in some situations.

Author: dljsjr

drivers/SmartThings/sonos/src/api/sonos_connection.lua

@@ -78,7 +78,11 @@ local _update_subscriptions_helper = function(
     local unique_key, bad_key_part = utils.sonos_unique_key(householdId, playerId)
     if not unique_key then
       local err_msg = string.format("Invalid Sonos Unique Key Part: %s", bad_key_part)
-      reply_tx:send(table.pack(nil, err_msg))
+      if reply_tx then
+        reply_tx:send(table.pack(nil, err_msg))
+      else
+        log.warn(string.format("Update Subscriptions Error: %s", err_msg))
+      end
       return
     end
     Router.send_message_to_player(unique_key, payload, reply_tx)