phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/widgets/desktop/shop_bulletin.dart

@@ -28,7 +28,7 @@ class ShopBulletinState extends State<ShopBulletin> {
       sideSpace = (MediaQuery.of(context).size.width - 1200) / 2;
     }
 
-    if (widget.business.bulletin != null && widget.business.bulletin.isNotEmpty) {
+    if (widget.business.bulletin != null && widget.business.bulletin!.isNotEmpty) {
       return Container(
         margin: EdgeInsets.only(bottom: 12),
         child: Row(