Do not drop the milestone slug in the URI of a 404

Summary:
Since rP70053bee and the followup fix in rP972788b8, Phorge does not generate slugs for milestones.

Users however can set an alternative slug for a milestone which can be used for typeahead autocompletion (writing `#alternative-milestone-slug` in a text field) but not to access a milestone by constructing an URI like `http://phorge.localhost/tag/alternative-milestone-slug/`.

Phorge correctly returns a 404 but also confusingly rewrites the URI to `http://phorge.localhost/tag//` as we have a valid `$slug` but our `$primary_slug` is `null`, and `$primary_uri = "/tag/{$primary_slug}/"` rewrites the URI accordingly. Instead, just keep the URI and throw an explicit 404.

Closes T15547

Test Plan: See the list of steps in T15547, look at the resulting URI.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15547

Differential Revision: https://we.phorge.it/D26124

src/applications/project/controller/PhabricatorProjectController.php

···
     // Redirect to the canonical slug.
     $primary_slug = $project->getPrimarySlug();
     if ($slug && ($slug !== $primary_slug)) {
+      // Things like milestones do not have a '/tag/something/' page.
+      if (!phutil_nonempty_string($primary_slug)) {
+        return new Aphront404Response();
+      }
       $primary_uri = "/tag/{$primary_slug}/";
       return id(new AphrontRedirectResponse())->setURI($primary_uri);
     }