Motivation

It's not obviously how to specify NULL value correctly.
Expected to have similar method to Expr::current_date for NULL, but there don't it.

In Expr::current_date function used such code: Self::Keyword(Keyword::CurrentDate), so we will try to use something like SimpleExpr::Keyword(Keyword::Null) according to expectations to write "similar code". It's too long and inconvenient.

There are also variant to use Keyword::Null directly, but docs doesn't show it and without read the code (From implementations for SimpleExpr), it will be difficult to find this variant.

Proposed Solutions

Add Expr::null method that will be friendly for users