Update perp position to return zero on nonexistent position

src/driftpy/drift_user.py

@@ -142,6 +142,9 @@ def get_perp_market_liability(
         signed: bool = False,
     ):
         perp_position = self.get_perp_position(market_index)
+        if perp_position is None:
+            return 0
+
         return self.calculate_weighted_perp_position_liability(
             perp_position,
             margin_category,