Handle this failure gracefully

I completely misunderstood the code in the previous attempt and messed
this up (in #524 and #525).

thetagang/portfolio_manager.py

@@ -1301,6 +1301,7 @@ async def row_position_task(position: PortfolioItem) -> None:
                 position.contract.exchange = self.get_order_exchange()
                 buy_ticker = await self.ibkr.get_ticker_for_contract(
                     position.contract,
+                    required_fields=[],
                     optional_fields=[TickerField.MIDPOINT, TickerField.MARKET_PRICE],
                 )