[SPARK-17112][SQL] "select null" via JDBC triggers IllegalArgumentException in Thriftserver

## What changes were proposed in this pull request?

Currently, Spark Thrift Server raises `IllegalArgumentException` for queries whose column types are `NullType`, e.g., `SELECT null` or `SELECT if(true,null,null)`. This PR fixes that by returning `void` like Hive 1.2.

**Before**
```sql
$ bin/beeline -u jdbc:hive2://localhost:10000 -e "select null"
Connecting to jdbc:hive2://localhost:10000
Connected to: Spark SQL (version 2.1.0-SNAPSHOT)
Driver: Hive JDBC (version 1.2.1.spark2)
Transaction isolation: TRANSACTION_REPEATABLE_READ
Error: java.lang.IllegalArgumentException: Unrecognized type name: null (state=,code=0)
Closing: 0: jdbc:hive2://localhost:10000

$ bin/beeline -u jdbc:hive2://localhost:10000 -e "select if(true,null,null)"
Connecting to jdbc:hive2://localhost:10000
Connected to: Spark SQL (version 2.1.0-SNAPSHOT)
Driver: Hive JDBC (version 1.2.1.spark2)
Transaction isolation: TRANSACTION_REPEATABLE_READ
Error: java.lang.IllegalArgumentException: Unrecognized type name: null (state=,code=0)
Closing: 0: jdbc:hive2://localhost:10000
```

**After**
```sql
$ bin/beeline -u jdbc:hive2://localhost:10000 -e "select null"
Connecting to jdbc:hive2://localhost:10000
Connected to: Spark SQL (version 2.1.0-SNAPSHOT)
Driver: Hive JDBC (version 1.2.1.spark2)
Transaction isolation: TRANSACTION_REPEATABLE_READ
+-------+--+
| NULL  |
+-------+--+
| NULL  |
+-------+--+
1 row selected (3.242 seconds)
Beeline version 1.2.1.spark2 by Apache Hive
Closing: 0: jdbc:hive2://localhost:10000

$ bin/beeline -u jdbc:hive2://localhost:10000 -e "select if(true,null,null)"
Connecting to jdbc:hive2://localhost:10000
Connected to: Spark SQL (version 2.1.0-SNAPSHOT)
Driver: Hive JDBC (version 1.2.1.spark2)
Transaction isolation: TRANSACTION_REPEATABLE_READ
+-------------------------+--+
| (IF(true, NULL, NULL))  |
+-------------------------+--+
| NULL                    |
+-------------------------+--+
1 row selected (0.201 seconds)
Beeline version 1.2.1.spark2 by Apache Hive
Closing: 0: jdbc:hive2://localhost:10000
```

## How was this patch tested?

* Pass the Jenkins test with a new testsuite.
* Also, Manually, after starting Spark Thrift Server, run the following command.
```sql
$ bin/beeline -u jdbc:hive2://localhost:10000 -e "select null"
$ bin/beeline -u jdbc:hive2://localhost:10000 -e "select if(true,null,null)"
```

**Hive 1.2**
```sql
hive> create table null_table as select null;
hive> desc null_table;
OK
_c0                     void
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15325 from dongjoon-hyun/SPARK-17112.