OMEMO: doesn't work in MUCs if there is only one participant

Steps to reproduce

1. Create private, non-anonymous MUC room

2. Make sure you're the only one in it

3. Enable OMEMO encryption

4. Try to send the message

Expected result

Message has been sent and is visible on all devices.

Actual result

"To send an encrypted message, you have to first trust the fingerprint of your contact!" green error text appears.

But I'm already trusting all of my devices!

I think this is caused by my MUC room having only one participant - me.

For the record, a similar issue exists in Conversations: https://github.com/siacs/Conversations/issues/2768