[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.

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/SparkExecuteStatementOperation.scala

@@ -56,14 +56,11 @@ private[hive] class SparkExecuteStatementOperation(
   private var statementId: String = _
 
   private lazy val resultSchema: TableSchema = {
-    if (result == null || result.queryExecution.analyzed.output.size == 0) {
+    if (result == null || result.schema.isEmpty) {
       new TableSchema(Arrays.asList(new FieldSchema("Result", "string", "")))
     } else {
-      logInfo(s"Result Schema: ${result.queryExecution.analyzed.output}")
-      val schema = result.queryExecution.analyzed.output.map { attr =>
-        new FieldSchema(attr.name, attr.dataType.catalogString, "")
-      }
-      new TableSchema(schema.asJava)
+      logInfo(s"Result Schema: ${result.schema}")
+      SparkExecuteStatementOperation.getTableSchema(result.schema)
     }
   }
 
@@ -282,3 +279,13 @@ private[hive] class SparkExecuteStatementOperation(
     }
   }
 }
+
+object SparkExecuteStatementOperation {
+  def getTableSchema(structType: StructType): TableSchema = {
+    val schema = structType.map { field =>
+      val attrTypeString = if (field.dataType == NullType) "void" else field.dataType.catalogString
+      new FieldSchema(field.name, attrTypeString, "")
+    }
+    new TableSchema(schema.asJava)
+  }
+}

sql/hive-thriftserver/src/test/scala/org/apache/spark/sql/hive/thriftserver/SparkExecuteStatementOperationSuite.scala

@@ -0,0 +1,33 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.hive.thriftserver
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.types.{NullType, StructField, StructType}
+
+class SparkExecuteStatementOperationSuite extends SparkFunSuite {
+  test("SPARK-17112 `select null` via JDBC triggers IllegalArgumentException in ThriftServer") {
+    val field1 = StructField("NULL", NullType)
+    val field2 = StructField("(IF(true, NULL, NULL))", NullType)
+    val tableSchema = StructType(Seq(field1, field2))
+    val columns = SparkExecuteStatementOperation.getTableSchema(tableSchema).getColumnDescriptors()
+    assert(columns.size() == 2)
+    assert(columns.get(0).getType() == org.apache.hive.service.cli.Type.NULL_TYPE)
+    assert(columns.get(1).getType() == org.apache.hive.service.cli.Type.NULL_TYPE)
+  }
+}