[master < T914] Add command NULLIF for identifying nulls in LOAD CSV

[Josipmrden]

The NULLIF command used in LOAD CSV enables for users to parse the LOAD CSV in a way which treats characters which have the identical character sequence as the NULLIF character as nulls.

Usage:
LOAD CSV FROM './file.csv' WITH HEADER NULLIF 'N/A' AS row ...

[master < Task] PR

• Check, and update documentation if necessary
• Provide the full content or a guide for the final git message

To keep docs changelog up to date, one more thing to do:

• Write a release note here
  Added NULLIF command in the LOAD CSV clause to specify a character sequence which will treat identical values in the csv file as nulls
• Tag someone from docs team in the comments

[antoniofilipovic]

I think you need to add the keyword nullif on src/query/frontend/stripped_lexer_constants.hpp as mentioned on this Notion page.

[antoniofilipovic]

Small change, otherwise looks good