Enhance: Add NonValueTransformer Reverse Conversion with NAN_VALUE Replacement

[MooooCat]

Description

Added a new method replace_nan_value within the NonValueTransformer class to handle the replacement of specific "NAN_VALUE" placeholders with empty strings during the reverse conversion process.

This method scans all rows and columns in the DataFrame and performs the replacement, enhancing the usability of the reverse-converted data.

Motivation and Context

This change is required to address the issue of placeholder values ("NAN_VALUE") persisting in the data after reverse conversion, which can lead to inaccuracies and confusion.

By implementing this, we ensure that the data returned after reverse conversion is cleaner and more representative of the original, intended data structure.

How has this been tested?

The new method replace_nan_value has been tested locally with various DataFrame structures and datasets to ensure it correctly identifies and replaces "NAN_VALUE" placeholders across different scenarios.

Additionally, unit tests have been designed to include cases for this new functionality, verifying its integration and impact on the overall reverse conversion process.

Types of changes

• Maintenance (no change in code, maintain the project's CI, docs, etc.)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.