_log_connection_get_property() is a hack, as it cannot meaningfully print complex
properties. Also, it uses _nm_setting_get_property() which can only work with GObject
base properties.

Don't assert against _nm_setting_get_property() returning success. Eventually
we should replace _nm_setting_get_property() by something better. But for the moment,
it's fine to being unable to print a property value.