dnsdist: Prevent Lua bindings for backend from crashing on empty backend

dnsdist: Prevent Lua bindings for backend from crashing on empty backend

We currently return an empty `std::shared_ptr` when the backend is not
set (self-answered response, for example), and unfortunately LuaWrapper
is not smart enough to make that equivalent to `nil`, so testing whether
the backend is valid from Lua is not possible. While I would prefer to
fix that, this fix prevents us from crashing when calling the bindings
associated to a backend with an empty shared pointer.

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