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/environment.js

@@ -364,7 +364,7 @@ function init() {
             this.getHours(),
             this.getMinutes(),
             this.getSeconds());
-        return dt ? dt.format(format) : '';
+        return dt?.format(format) ?? '';
     };
 
     let slowdownEnv = GLib.getenv('GNOME_SHELL_SLOWDOWN_FACTOR');