rigol-ds: prevent config_list() to return empty g_variant

This fixes the following glib message:
$ ./sigrok-cli -d rigol-ds --show
[...]
g_variant_builder_end: assertion '!GVSB(builder)->uniform_item_types || GVSB(builder)->prev_item_type != NULL || g_variant_type_is_definite (GVSB(builder)->type)' failed
rigol-ds: remove lonely break