[SPARK-3004][SQL] Added null checking when retrieving row set

JIRA issue: [SPARK-3004](https://issues.apache.org/jira/browse/SPARK-3004)

HiveThriftServer2 throws exception when the result set contains `NULL`. Should check `isNullAt` in `SparkSQLOperationManager.getNextRowSet`.

Note that simply using `row.addColumnValue(null)` doesn't work, since Hive set the column type of a null `ColumnValue` to String by default.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1920 from liancheng/spark-3004 and squashes the following commits:

1b1db1c [Cheng Lian] Adding NULL column values in the Hive way
2217722 [Cheng Lian] Fixed SPARK-3004: added null checking when retrieving row set