src/config_file: allow empty description to be NULL
src/config_file: allow empty description to be NULL

Otherwise all handling relying on checking if slot->description is NULL
is useless and will always print out description with an empty string
instead of hiding it.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc