Added CSV Null Representation Setting for Output Format

Why it matters

This feature addresses the need to customize the representation of NULL values when exporting data in CSV format, similar to the existing capability for TSV format with output_format_tsv_null_representation. It allows users to better control and standardize the output to meet their data processing or integration requirements.

How to use it

Set the output_format_csv_null_representation setting to the desired string value before exporting data with CSV format. For example, use SET output_format_csv_null_representation = 'NULL'; to represent NULL values as the string "NULL" in the CSV output.