BUG: Fix empty MultiIndex DataFrame xlsx export

MultiIndex on both axes, with zero rows.

Fixes pandas-dev#57696

Author: speleo3

pandas/io/formats/excel.py

@@ -620,7 +620,7 @@ def _format_header_mi(self) -> Iterable[ExcelCell]:
         lnum = 0
 
         if self.index and isinstance(self.df.index, MultiIndex):
-            coloffset = len(self.df.index[0]) - 1
+            coloffset = self.df.index.nlevels - 1
 
         if self.merge_cells:
             # Format multi-index as a merged cells.

pandas/tests/io/excel/test_writers.py

@@ -936,6 +936,16 @@ def test_to_excel_empty_multiindex(self, tmp_excel):
             result, expected, check_index_type=False, check_dtype=False
         )
 
+    def test_to_excel_empty_multiindex_both_axes(self, tmp_excel):
+        df = DataFrame(
+            [],
+            index=MultiIndex.from_tuples([], names=[0, 1]),
+            columns=MultiIndex.from_tuples([("A", "B")]),
+        )
+        df.to_excel(tmp_excel)
+        result = pd.read_excel(tmp_excel, header=None)
+        assert result.iloc[:2, -1].to_list() == ["A", "B"]
+
     def test_to_excel_float_format(self, tmp_excel):
         df = DataFrame(
             [[0.123456, 0.234567, 0.567567], [12.32112, 123123.2, 321321.2]],