check empty before make set

Author: quangngd

pandas/core/reshape/reshape.py

@@ -929,9 +929,9 @@ def _reorder_for_extension_array_stack(
 def stack_v3(frame: DataFrame, level: list[int]) -> Series | DataFrame:
     if frame.columns.nunique() != len(frame.columns):
         raise ValueError("Columns with duplicate values are not supported in stack")
-    set_levels = set(level)
-    if not len(set_levels):
+    if not len(level):
         return frame
+    set_levels = set(level)
     stack_cols = frame.columns._drop_level_numbers(
         [k for k in range(frame.columns.nlevels - 1, -1, -1) if k not in set_levels]
     )