Fix 2499d7b to not break user registration.

Some bots created by us do not have owners. Don't try to send a
message to the nonexistent owner.

(imported from commit ab952eccd7d6c4728e9477a106142214b5c81ca9)

zerver/lib/actions.py

@@ -449,7 +449,7 @@ def check_message(sender, client, message_type_name, message_to,
 
         stream = get_stream(stream_name, realm)
 
-        if sender.is_bot:
+        if sender.is_bot and sender.bot_owner is not None:
             if stream:
                 num_subscribers = len(maybe_get_subscriber_emails(stream))