loader: Fix loader_brand = none

Setting loader_brand to none in loader.conf now result in a lua error
as we try to get a non-existant table
Defines it

Differential Revision: https://reviews.freebsd.org/D50875
Fixes: bef6d85b6de5 ("lualoader: allow graphical bits to be disabled with loader_gfx")
Reviewed by: kevans, tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG