unknown user url redirect loop

fixes: CNVS-15618

Checks for a blank unknown_user_url along with a nil before defaulting
to the login_url.

Test Plan:

  - Setup SAML or CAS with a user that does not exist in Canvas.
  - Make sure the unknown_user_url is an empty string for the accounts
    authorization config.
  - Attempt to login to the user with and it should show a Canvas login
    page with an error that the user does not exist.
  - Change the unknown_user_url to nil and it should behave the same as
    if it was an empty string.
  - Change the unknown_user_url to a custom url and it should redirect
    to that page instead of showing the Canvas login page with an error.

Change-Id: I3d8de694100e2849119adfc3b0dd08ead58a80ed
Reviewed-on: https://gerrit.instructure.com/41296
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>