fix(auth): fix authentification issue when user has no avatar

[Akinator31]

This pull request updates the handling of Discord user avatars, allowing for users without a custom avatar to be supported. The migration modifies the database schema to allow avatar_hash to be nullable, and the API handler logic now correctly generates a default avatar URL when no custom hash is present.

Database schema update:

• Modified the sessions_panel table to allow avatar_hash to be NULL, enabling support for users without a Discord avatar.

API handler improvements:

• Updated the avatar retrieval logic in handle_get_user_avatar to treat avatar_hash as an Option<String>, handling cases where the value is missing.
• Added logic to generate a default Discord avatar URL when avatar_hash is None, based on the user ID.