Fix fatal during redirection safety check for searching for Phabricator base-uri with no trailing slash

Summary: Fixes T13412. If you search for "https://phabricator.example.com" with no trailing slash, we currently redirect you to "", which is fouled by a safety check in the redirection flow.

Test Plan:
  - Searched for "https://local.phacility.com"; before: fatal in redirection; after: clean redirect.
  - Searched for other self-URIs, got normal redirects.

Maniphest Tasks: T13412

Differential Revision: https://secure.phabricator.com/D20794

src/applications/meta/engineextension/PhabricatorDatasourceURIEngineExtension.php

@@ -26,7 +26,17 @@ final class PhabricatorDatasourceURIEngineExtension
         ->setProtocol(null)
         ->setPort(null);
 
-      return phutil_string_cast($uri);
+      $uri = phutil_string_cast($uri);
+
+      // See T13412. If the URI was in the form "http://dev.example.com" with
+      // no trailing slash, there may be no path. Redirecting to the empty
+      // string is considered an error by safety checks during redirection,
+      // so treat this like the user entered the URI with a trailing slash.
+      if (!strlen($uri)) {
+        $uri = '/';
+      }
+
+      return $uri;
     }
 
     return null;