update scalable_sync.py to modify WealthfolioActivity symbol field to be optional and set default to None for improved flexibility

scalable_sync.py

@@ -185,7 +185,7 @@ def mark_seen(tx_id, tx_hash, status):
 @dataclass
 class WealthfolioActivity:
     date: str
-    symbol: str
+    symbol: Optional[str] = None
     activity_type: str
     quantity: Optional[float] = None
     unit_price: Optional[float] = None
@@ -267,7 +267,7 @@ def map_to_wealthfolio(tx: dict) -> list[WealthfolioActivity]:
 
         activities.append(WealthfolioActivity(
             date=last_event,
-            symbol="$CASH",
+            symbol=None,
             activity_type=wf_type,
             amount=amount,
             currency=tx.get("currency", "EUR"),