BUG: Fix inconsistency when read_csv reads MultiIndex with empty values (#59560)

[allamlobna]

• closes BUG: inconsistency when read_csv reads MultiIndex with empty values #59560
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Summary

This PR fixes an inconsistency where read_csv replaced empty
MultiIndex column level values with automatically generated
"Unnamed: x_level_y" placeholders.

Empty values are now preserved as empty strings (""),
matching MultiIndex index behavior and ensuring consistent
roundtrip results between to_csv and read_csv.

Changes

• Added _clean_column_levels() helper to normalize MultiIndex
  column labels in BaseParser.
• Updated _extract_multi_indexer_columns() to use it.
• Adjusted test_multi_index_unnamed() expectations.
• Added regression test for GH#59560 in test_header.py.
• Added whatsnew entry under Bug Fixes → IO.

Impact

• Aligns column + index behavior for MultiIndex CSVs.
• No change for single-level headers.
• Both C and Python parsers tested successfully.