Function sql_null_test 

pub fn sql_null_test(value: Option<&Value>, negated: bool) -> bool

SQL IS NULL (or IS NOT NULL when negated), as PostgreSQL’s NullTest evaluates it: a row value is null when every field is null and not null when no field is, so a row with both kinds of fields is neither; any other value is null only when it is the null value. An absent field is null.