When schema change and compaction is executing simutaneously, both nullable and not nullable data can be read for the same column, need to reset _nullmap for each Block when converting Block data, or else Column case will be wrong.
When schema change and compaction is executing simutaneously, both nullable and not nullable data can be read for the same column, need to reset _nullmap for each Block when converting Block data, or else Column case will be wrong.