[SPARK-11997] [SQL] NPE when save a DataFrame as parquet and partitioned by long column

Check for partition column null-ability while building the partition spec.

Author: Dilip Biswal <dbiswal@us.ibm.com>

Closes #10001 from dilipbiswal/spark-11997.

sql/core/src/main/scala/org/apache/spark/sql/sources/interfaces.scala

@@ -607,7 +607,7 @@ abstract class HadoopFsRelation private[sql](
         def castPartitionValuesToUserSchema(row: InternalRow) = {
           InternalRow((0 until row.numFields).map { i =>
             Cast(
-              Literal.create(row.getString(i), StringType),
+              Literal.create(row.getUTF8String(i), StringType),
               userProvidedSchema.fields(i).dataType).eval()
           }: _*)
         }

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetQuerySuite.scala

@@ -252,6 +252,19 @@ class ParquetQuerySuite extends QueryTest with ParquetTest with SharedSQLContext
     }
   }
 
+  test("SPARK-11997 parquet with null partition values") {
+    withTempPath { dir =>
+      val path = dir.getCanonicalPath
+      sqlContext.range(1, 3)
+        .selectExpr("if(id % 2 = 0, null, id) AS n", "id")
+        .write.partitionBy("n").parquet(path)
+
+      checkAnswer(
+        sqlContext.read.parquet(path).filter("n is null"),
+        Row(2, null))
+    }
+  }
+
   // This test case is ignored because of parquet-mr bug PARQUET-370
   ignore("SPARK-10301 requested schema clipping - schemas with disjoint sets of fields") {
     withTempPath { dir =>