Bug 30979: (follow-up) Fix public availability 500

Bug 30979: (follow-up) Fix public availability 500

When we introduced the logged in user id to the token generation we
had the knock on effect of creating a 500 response for unauthenticated
availability lookups.

This patch resolves the 500, but results in a token that's not useful
for the end user.  We should consider this in the wider context down the
line.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>