[SPARK-9101] [PySpark] Add missing NullType

JIRA: https://issues.apache.org/jira/browse/SPARK-9101

Author: Mateusz Buśkiewicz <mateusz.buskiewicz@getbase.com>

Closes #7499 from sixers/spark-9101 and squashes the following commits:

dd75aa6 [Mateusz Buśkiewicz] [SPARK-9101] [PySpark] Test for selecting null literal
97e3f2f [Mateusz Buśkiewicz] [SPARK-9101] [PySpark] Add missing NullType to _atomic_types in pyspark.sql.types

python/pyspark/sql/tests.py

@@ -333,6 +333,10 @@ class SQLTests(ReusedPySparkTestCase):
         df = self.sqlCtx.inferSchema(rdd)
         self.assertEquals(Row(field1=1, field2=u'row1'), df.first())
 
+    def test_select_null_literal(self):
+        df = self.sqlCtx.sql("select null as col")
+        self.assertEquals(Row(col=None), df.first())
+
     def test_apply_schema(self):
         from datetime import date, datetime
         rdd = self.sc.parallelize([(127, -128, -32768, 32767, 2147483647, 1.0,

python/pyspark/sql/types.py

@@ -642,7 +642,7 @@ class UserDefinedType(DataType):
 
 
 _atomic_types = [StringType, BinaryType, BooleanType, DecimalType, FloatType, DoubleType,
-                 ByteType, ShortType, IntegerType, LongType, DateType, TimestampType]
+                 ByteType, ShortType, IntegerType, LongType, DateType, TimestampType, NullType]
 _all_atomic_types = dict((t.typeName(), t) for t in _atomic_types)
 _all_complex_types = dict((v.typeName(), v)
                           for v in [ArrayType, MapType, StructType])