Fix guest avatar in device checker
Fix guest avatar in device checker

"firstLetterOfGuestName" was not defined, and the "avatar" CSS class
needs to include the size to be properly shown.

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed