This was a special feature for a project I was working on long ago.
It provided implicit behavior for trivial value (0 or "") which were
supposed to be interpreted as NULL.
They led to `operator==` potentially being serialized as 'IS NULL'.
It makes more sense to introduce explicit helpers, e.g.
- equal_or_is_null(col, optional)
- not_equal_or_is_not_null(col, optional)