[2pt] Allow update absent nullable fields

Allow update absent nullable fields

Product: Tarantool
Since: 2.8.1
Audience/target: all Tarantool users
Root document: http://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/update/
SME: @mary3000
Peer reviewer:

Details

Previously update operation could not insert with gaps. Changed the behavior
so that the update operation fills the missing fields with nulls.

For example we create space s = box.schema.create_space('s'),
then create index for this space pk = s:create_index('pk'), and
then insert tuple in space s:insert{1, 2}. After all of this we
try to update this tuple s:update({1}, {{'!', 5, 6}}). In previous
version this operation fails with ER_NO_SUCH_FIELD_NO error, and now
it's finished with success and there is [1, 2, null, null, 6] tuple in
space.

Related commit: https://github.com/tarantool/tarantool/commit/2bb373b9f5589cbda667f3d7465aae23075e49bc

Definition of done

• New behavior of update operation is described.

Open the linked update operation reference document and review commit 2bb373b9f5589cbda667f3d7465aae23075e49bc for context. Describe that updates can fill absent fields with nulls, include the provided example and resulting tuple, and verify the documentation matches the definition of done.