feat: Null keyword Completeness validation function

[subhankarb]

Tell us about the problem you're trying to solve

As a user, I want to test the column where the text value is a null keyword ("none", "null", "nil", "nothing" or "n/a")

Describe the solution you’d like

A total of two all-space functions need to be implemented:

• COUNT_NULL_KEYWORD
• PERCENT_NULL_KEYWORD

Example:

validations for product_db.products:

• max 10 rows should have null keyword:
  on: count_null_keyword(price)
  threshold: <= 10