g_object_class_list_properties() can't return NULL if called
correctly.

Also remove two failed attempts to use g_value_transform():
nm_setting_new_from_hash() was transforming src_value to its own type
(rather than to param_spec->value_type, which was presumably
intended), so it was a no-op (in addition to being unnecessary anyway,
since GObject will attempt to transform the value internally if
needed). And update_one_secret() was calling g_value_transform() on an
uninitialized GValue, so it would have always hit a
g_return_val_if_fail() in g_value_transform() if that code was ever
reached (which apparently it wasn't).