Fix to_excel not rendering None values in MultiIndex

[ghost]

• closes BUG: to_excel inserts invalid data if a level in a MultiIndex is None #51252
• 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.

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The change fixes the issue of not rendering NaN-valued column cells when exported to Excel.

import pandas as pd
import numpy as np

Cols = pd.MultiIndex.from_tuples(
    [
        ("Item", "Type 1", "Class A"),
        ("Item", "Type 1", "Class B"),
        ("Item", "Type 2", "Class A"),
        ("Item", "Type 2", "Class B"),
        ("Item", "Type 3", None),
    ]
)

df = pd.DataFrame(np.random.randn(10, 5), columns=Cols)
df.to_excel("Test.xlsx")
df

However, when I do roundtripping of the Excel sheet back into a DataFrame I get this:

pd.read_excel("Test.xlsx", header=[0, 1, 2], index_col=[0])

So I have a question. Should I update the logic of read_excel to also apply the conversion of NaN values of regular cells (specified by the na_values parameter) to column values as well? If so, would it make more sense to do it here (where regular values are converted), or here (where column values assigned the Unnamed values)? Or there's some reason why the current functionality is not applied for column values and the user is expected to convert them manually (like here)?

[rhshadrach]

I don't believe we can differentiate between '' and an NA value in the Excel file. Would it make more sense to assume the value is '' instead of an NA value?

[mroeschke]

Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.