Bugfix/remove error log of anonymous access

Original issue #876 

- Removed `no_user_key` error log 152793b
- Changed handling of `undefind` for user key to avoid error messages like
  `Retrieval of user record for [] failed. Reason: <<"Key cannot be zero-length">>' .
  These logs happened when access admin resources `/riak-cs/users` as anonymous
  with configuration `{admin_auth_enabled, false}`.