[CH-436] support columns with different nullable type when split for union

[shuai-xu]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in official stable or prestable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

This pr supports that when split blocks, they may have differnt nullable types as they may come from two different steams of union. It fix #436 .

[kyligence-git]

Can one of the admins verify this patch?

[lgbo-ustc]

The main problem here is that, the 1st block comed into ColumnBuffer could be non-nullable, but the later blocks come with nullable, and the real final schema shoul be nullable.

You can make an assumption that, if we meet a nullable column once, all results should be nullable. but we may have spill some blocks with non-nullable columns out into next stage before we meet the first block with nullable.