🖼️ Conversation avatars API - Follow up

[nickvergessen]

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Follow up to #8333

Steps to reproduce

1. Upload an avatar to a conversation
2. Change an existing avatar of a conversation
3. Delete the avatar of a conversation

curl -k -H "OCS-APIRequest: true" -u admin:admin "https://nextcloud.local/ocs/v2.php/apps/spreed/api/v1/room/am2f9ye3/avatar" -X POST -F 'file=@/home/…'

curl -k -H "OCS-APIRequest: true" -u admin:admin "https://nextcloud.local/ocs/v2.php/apps/spreed/api/v1/room/am2f9ye3/avatar" -X POST -F 'file=@/home/…'

curl -k -H "OCS-APIRequest: true" -u admin:admin "https://nextcloud.local/ocs/v2.php/apps/spreed/api/v1/room/am2f9ye3/avatar" -X DELETE

Expected behaviour

Admin set the conversation avatar
Admin set the conversation avatar
Admin deleted the conversation avatar

Actual behaviour

--- Chat is empty ---