sysregistriesv2: do not require default config to exist
sysregistriesv2: do not require default config to exist

When using the default config, remove the requirement for it to exist,
unless being specified in the SystemContext, and return an empty
Registry array instead.

Signed-off-by: Valentin Rothberg <6db4ac3d4f2c7b0d4923f1257f87bfb6a65f568b@suse.com>

containers/image,containers/image