[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.