Function nullify_fields

datafusion/expr/src/logical_plan/builder.rs:1667–1677

(
        fields: impl Iterator<Item = (Option<&'a TableReference>, &'a Arc<Field>)>,
    )

    join_type: &JoinType,
) -> Result<DFSchema> {
    fn nullify_fields<'a>(
        fields: impl Iterator<Item = (Option<&'a TableReference>, &'a Arc<Field>)>,
    ) -> Vec<(Option<TableReference>, Arc<Field>)> {
        fields
            .map(|(q, f)| {
                // TODO: find a good way to do that
                let field = f.as_ref().clone().with_nullable(true);
                (q.cloned(), Arc::new(field))
            })
            .collect()
    }

    let right_fields = right.iter();
    let left_fields = left.iter();