Fix chat endpoints with empty reactions in XML format

[nickvergessen]

Fix #7952

Check result of both curl commands before and after:

# json
curl -k -u admin:admin \
    'https://nextcloud25.local/ocs/v2.php/apps/spreed/api/v1/chat/7d85h4a9?setReadMarker=0&lookIntoFuture=0&lastKnownMessageId=201&includeLastKnown=1' \
    -H 'OCS-APIRequest: true' \
    -H 'Accept: application/json, text/plain, */*' \
    | jq .


# xml
curl -k -u admin:admin \
    'https://nextcloud25.local/ocs/v2.php/apps/spreed/api/v1/chat/7d85h4a9?setReadMarker=0&lookIntoFuture=0&lastKnownMessageId=201&includeLastKnown=1' \
    -H 'OCS-APIRequest: true'

(Adjust domain and token and make sure the json one still returns an empty object for reactions

[nickvergessen]

cc @meonkeys

[nickvergessen]

/backport to stable24