[Backtesting] fix futures backtesting ref market

Author: Guillaume De Saint Martin

octobot/backtesting/independent_backtesting.py

@@ -370,7 +370,9 @@ def _find_reference_market(self):
                         self.logger.error(f"Mixed inverse and linear contracts backtesting are not supported yet")
                     self.octobot_backtesting.futures_contract_type = trading_enums.FutureContractType.LINEAR_PERPETUAL
                 # in inverse contracts, use BTC for BTC/USD trading as reference market
-                return symbol.settlement_asset
+                if symbol.settlement_asset:
+                    # only use settlement asset if available
+                    return symbol.settlement_asset
             for symbol in symbols:
                 quote = symbol.quote
                 if ref_market_candidate is None: