fix: Correct logic for sending broadcast messages when mismatch happens

[atomrc]

When a message is broadcasted (without a conversation param), we need to create sessions for the clients we don't have a session with.
The previous return false would stop sending the message if there is a mismatch. Which is not the intended behaviour.

see https://wearezeta.atlassian.net/wiki/spaces/ENGINEERIN/pages/106660748/Use+case+setting+the+status#Step-2 for specs

If the backend didn't accept the message but returned a 412 missing clients, the response will contain a list of missing clients that were not included as recipients of the original message.

The client should create a session with those clients (which includes fetching prekeys) and then resend the message.