Do not allow participants w/ empty name

[ThiefMaster]

Not sure how it happened in the first place and I'm pretty sure we reject it AT LEAST on the frontend, but if you end up with such a participant in the DB, it fails when trying to generate the avatar url:

File "/usr/local/lib/python3.12/site-packages/newdle/schemas.py", line 130, in <lambda>
'api.user_avatar', payload=avatar_payload_from_participant(participant)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/newdle/core/util.py", line 123, in avatar_payload_from_participant
'initial': participant.name[0].upper(),
~~~~~~~~~~~~~~~~^^^
IndexError: string index out of range

[renefs]

I can take a look if you want