cleanup: Use optional chaining and ?? operator

Those operators have been supported since gjs switched to mozjs78
last cycle. While not ground-breaking, using it makes for a nice
cleanup here and there.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1517>

js/ui/components/networkAgent.js

@@ -793,7 +793,7 @@ var NetworkAgent = class {
         }
 
         const prop = plugin.lookup_property('GNOME', 'supports-external-ui-mode');
-        const trimmedProp = prop ? prop.trim().toLowerCase() : '';
+        const trimmedProp = prop?.trim().toLowerCase() ?? '';
 
         return {
             fileName,