ALTER TABLE ADD COLUMN should default to NULL (instead of NOT NULL)

Currently, the following sequence results in an error:

create table foo(id int primary key);
insert into foo values (1);
alter table foo add column val int;

ddl operation was aborted: [instance name:default_1_1] FieldMissing: box error: FieldMissing: Tuple field 3 (val) required by space format is missing

This is due to alter table foo add column val int; being interpreted as alter table foo add column val int NOT NULL;. However, this is different from postgres or mysql and works worse with concurrent DML.

We should reverse the default to NULL instead.