Start using the new JSON-GLib functions "*_with_default"
JSON-Glib 1.6.0 introduced new functions to get members from a JSON object with a default fallback value. This allows us to avoid manually checking the existence of a member.
The functions that we currently need have been backported to allow the execution on systems with a JSON-GLIB version older than the 1.6.0
/cc @smcv