Fix 500 Error for anonymous users to visit flow start page

Sorry, I found one Wrinkle with !24 (merged): visits of anonymous users to flow start page always result in no session_grading_rule, which consequently lead to RuntimeError(_("grading rule determination was unable to find a grading rule")).

The reason is if an anonymous user is visiting the start page, _eval_participation_tags_conditions will always return False. The work around is, only return False for anonymous users if only there's a valid if_has_participation_tags_{all,any}.