Is safe adding uid=0 in users table just to use $_SESSION?
Drupal does come with a row with uid=0 by default.

Some backup tools (notoriously some versions of PHPMyAdmin) are unable to deal with that, so you will lose this row when restoring from backup.

The easiest way to get back the missing row is to issue this query (for example in PHPMyAdmin):

UPDATE users SET uid = uid - uid WHERE name = ''