Don't show errors when getting nonexistent settings

[aaronfranke]

Follow-up fix to #31171 and #55303. I was focused on getting it working last night but I forgot to check the console for errors. The current master does work except for the fact that it shows these errors:

ERROR: Property not found: application/config/features
   at: _get (core/config/project_settings.cpp:320)
ERROR: Property not found: rendering/quality/driver/driver_name
   at: _get (core/config/project_settings.cpp:320)

(btw, get_setting and set_setting seem to just be simple wrappers for get and set, but it's definitely good to use these for consistency with has_setting since there is no "has" without "_setting").

[akien-mga]

Thanks!