Fix sending note to self as primary without secondary devices

[gferon]

There might be a better way to do this but... here's what happens:

1. Register a client as primary (no extra devices)
2. Try to send a message to self which will try to call create_encrypted_message but will never send any messages because we purposefully skip sending messages to ourselves (and own device) for good reasons
3. At the very end of the process, we fetch the identity_key (public key) we have for the recipient, which fails because we've never established a session with ourselves (this would only happen if we have other devices).

Here are two improvements:

• When the result of create_encrypted_messages return 0 messages, do nothing
• ...and check that we're is_multi_device before even attempting to craft and send a SyncMessage to self (that was missed when doing Only send a SyncMessage when sending to self #338)