conf: Change handling for empty NVRAM path

conf: Change handling for empty NVRAM path

Right now, this results in loader->nvram being NULL, which is
reasonable: loader->nvramTemplate is stored separately, so if
the <nvram> element doesn't contain a path there is really no
useful information inside it.

However, this is about to change, so we will find ourselves
needing to hold on to loader->nvram even when no path is
present. Change the firmware handling code so that such a
scenario is dealt with appropriately.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>