Shares in federated conversation broken for public conversations

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Steps to reproduce

1. Instance A: Have a conversation that is public (!)
2. Instance A: Post a picture
3. Instance A: Invite participant from instance B
4. Instance B: Accept invite and open conversation

Expected behaviour

Actual behaviour

From what I can see it's a regression from conversation folders, namely https://github.com/nextcloud/spreed/commit/bb1a36d830255f046e96cbbeac324b4595e8aeac.

https://github.com/nextcloud/spreed/blob/c22c37107ae58d690ddae86065c011579e2851fc/lib/Chat/Parser/SystemMessage.php#L544-L550

Since this is not throwing anymore, we don't end up at https://github.com/nextcloud/spreed/blob/c22c37107ae58d690ddae86065c011579e2851fc/lib/Chat/Parser/SystemMessage.php#L575-L577

Talk app

Talk app version: Tested stable34 -> main

Start in lib/Chat/Parser/SystemMessage.php at the cited lines 544-550 and 575-577, then compare the behavior with the conversation-folders regression commit bb1a36d. Reproduce the public federated conversation flow with an image between two instances and verify that the shared image reaches the expected state shown in the issue.