Fix alignment in Series subtraction with MultiIndex, Index and NaN values (#60908)
This pull request fixes #60908 , where subtracting a Series with a MultiIndex containing NaN values from a Series with a regular Index could lead to incorrect results or unexpected behavior.

The issue was caused by the _align_for_op method not properly handling cases where the left-hand Series had a MultiIndex and the right-hand side had a flat Index, especially when NaN values were present. This could lead to misalignment during arithmetic operations.

To fix this, the _align_for_op method was updated to:

- Ensure that when the left Series has a MultiIndex and the right Series has a regular Index, the right Series is properly reindexed based on the first level of the left-hand MultiIndex, even when NaN values are involved.

- Correctly handle cases where either Series is empty.

Additionally, a new test test_series_subtraction_with_nan_and_levels (added in test_subtraction_nanindex) was introduced to verify that:

- Subtracting a Series with a MultiIndex (including NaNs) from a regular Index works correctly.

- The result maintains the correct alignment and expected output values.

Tested with x86-64 linux. Doesn’t reproduce on Linux-32-bit.
Hi, just following up on this PR. Let me know if you have any feedback or if anything is needed from my side. Thank you This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this.