Trying to retrieve a non existing slug results in an internal server error

When a non-existing slug is being searched for, an internal server error occurs (HTTP 500).

Probably due to a collection being `null`.

Start by reproducing a lookup for a non-existing slug in the Azure Functions application and trace the request path that produces HTTP 500 when the collection is null. Done means the missing-slug request no longer returns an internal server error and instead has an appropriate not-found response.

Written by the indexing model from the issue text.