[CH-426]fix null partition read

[lhuang09287750]

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):

fix partition column read for null value

Documentation entry for user-facing changes

Spark+ch backend will read the null partition column as the default value of partition column type and this behavior is differrent from spark which will read the null just as null. So this pr add a fix to process null partition column read problem.

[kyligence-git]

Can one of the admins verify this patch?

[lhuang09287750]

https://github.com/Kyligence/ClickHouse/issues/426, that is the issue which contains a test case

[lhuang09287750]

@liuneng1994 CI still in problem?