chore: add nulls_first to visit sort keys in pandas executor

ibis/backends/pandas/executor.py

@@ -69,7 +69,7 @@ def visit(cls, op: ops.Alias, arg, name):
             return arg
 
     @classmethod
-    def visit(cls, op: ops.SortKey, expr, ascending):
+    def visit(cls, op: ops.SortKey, expr, ascending, nulls_first):
         return expr
 
     @classmethod