dnsdist: Fix access to frontends while in client mode

dnsdist: Fix access to frontends while in client mode

Since 2.0 we return `nil` instead of an object containing a `NULL`
pointer when the requested object does not exist, to make it possible
to check the validity of the returned object from `Lua`. It makes
sense in all contexts except when we are in client mode, because
then accessing the object in the remaining parts of the configuration
will trigger an error. Our DNS over HTTPS documentation itself contains
such a Lua configuration snippet, which is now broken.
This commit reverts back to sending an object containg a `NULL`
pointer when accessing the frontends in the client mode case.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>