Fix for pandas issue pandas-dev#56929, including new test

Author: CYHSM

pandas/_libs/parsers.pyx

@@ -746,7 +746,7 @@ cdef class TextReader:
                               is not None else 0)
 
                         # if wrong number of blanks or no index, not our format
-                        if (lc != unnamed_count and lc - ic > unnamed_count) or ic == 0:
+                        if (lc != unnamed_count and lc - ic >= unnamed_count) or ic == 0:
                             hr -= 1
                             self.parser_start -= 1
                             this_header = [None] * lc

pandas/tests/io/formats/test_to_csv.py

@@ -332,6 +332,22 @@ def test_to_csv_float_ea_no_float_format(self):
         )
         assert result == expected
 
+    def test_to_csv_multi_index_nan(self):
+        # Create a MultiIndex DataFrame
+        columns = pd.MultiIndex.from_tuples([('Level 1', 'Level 2')], names=['level1', 'level2'])
+        data = [[np.nan], [0.1], [0.4]]
+        df_complex = pd.DataFrame(data, columns=columns)
+
+        # Expected DataFrame
+        expected_df = pd.DataFrame(data, columns=columns, index=range(3))
+
+        # Save and load the DataFrame as a CSV
+        with tm.ensure_clean("complex_data.csv") as path:
+            df_complex.to_csv(path)
+            loaded_df_complex = pd.read_csv(path, header=[0, 1], index_col=0)
+
+        tm.assert_frame_equal(loaded_df_complex, expected_df)
+
     def test_to_csv_multi_index(self):
         # see gh-6618
         df = DataFrame([1], columns=pd.MultiIndex.from_arrays([[1], [2]]))