[CH-422] Fix NPE caused by CHColumnToSparkRow when converting map field

[exmy]

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 NPE caused by CHColumnToSparkRow when converting map field, close #422

[kyligence-git]

Can one of the admins verify this patch?