On second thought, completely remove this check.

If the admin (unwisely) is not caching, that's their prerogative. That
shouldn't change where large items should be stored though (if cache
driver is null, they simply won't be cached).

framework/Core/lib/Horde/Core/Cache/Session.php

@@ -37,8 +37,8 @@ class Horde_Core_Cache_Session extends Horde_Cache_Storage_Base
     /**
      * @param array $params  Configuration parameters:
      *   - app: (string) Application to store session data under.
-     *   - cache: (Horde_Cache_Storage_Backend) [REQUIRED] The backend cache
-     *            storage driver used to store large entries.
+     *   - cache: (Horde_Cache) [REQUIRED] The backend cache driver used to
+     *            store large entries.
      *   - maxsize: (integer) The maximum size of the data to store in the
      *              session (0 to always store in session).
      *   - storage_key: (string) The storage key to save the session data
@@ -58,10 +58,6 @@ public function __construct(array $params = array())
             ),
             $params
         ));
-
-        if ($params['cache'] instanceof Horde_Cache_Storage_Null) {
-            $this->_params['maxsize'] = 0;
-        }
     }
 
     /**