SPARK-1757 Failing test for saving null primitives with .saveAsParquetFile()

https://issues.apache.org/jira/browse/SPARK-1757

The first test succeeds, but the second test fails with exception:

```
[info] - save and load case class RDD with Nones as parquet *** FAILED *** (14 milliseconds)
[info]   java.lang.RuntimeException: Unsupported datatype StructType(List())
[info]   at scala.sys.package$.error(package.scala:27)
[info]   at org.apache.spark.sql.parquet.ParquetTypesConverter$.fromDataType(ParquetRelation.scala:201)
[info]   at org.apache.spark.sql.parquet.ParquetTypesConverter$$anonfun$1.apply(ParquetRelation.scala:235)
[info]   at org.apache.spark.sql.parquet.ParquetTypesConverter$$anonfun$1.apply(ParquetRelation.scala:235)
[info]   at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
[info]   at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
[info]   at scala.collection.immutable.List.foreach(List.scala:318)
[info]   at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
[info]   at scala.collection.AbstractTraversable.map(Traversable.scala:105)
[info]   at org.apache.spark.sql.parquet.ParquetTypesConverter$.convertFromAttributes(ParquetRelation.scala:234)
[info]   at org.apache.spark.sql.parquet.ParquetTypesConverter$.writeMetaData(ParquetRelation.scala:267)
[info]   at org.apache.spark.sql.parquet.ParquetRelation$.createEmpty(ParquetRelation.scala:143)
[info]   at org.apache.spark.sql.parquet.ParquetRelation$.create(ParquetRelation.scala:122)
[info]   at org.apache.spark.sql.execution.SparkStrategies$ParquetOperations$.apply(SparkStrategies.scala:139)
[info]   at org.apache.spark.sql.catalyst.planning.QueryPlanner$$anonfun$1.apply(QueryPlanner.scala:58)
[info]   at org.apache.spark.sql.catalyst.planning.QueryPlanner$$anonfun$1.apply(QueryPlanner.scala:58)
[info]   at scala.collection.Iterator$$anon$13.hasNext(Iterator.scala:371)
[info]   at org.apache.spark.sql.catalyst.planning.QueryPlanner.apply(QueryPlanner.scala:59)
[info]   at org.apache.spark.sql.SQLContext$QueryExecution.sparkPlan$lzycompute(SQLContext.scala:264)
[info]   at org.apache.spark.sql.SQLContext$QueryExecution.sparkPlan(SQLContext.scala:264)
[info]   at org.apache.spark.sql.SQLContext$QueryExecution.executedPlan$lzycompute(SQLContext.scala:265)
[info]   at org.apache.spark.sql.SQLContext$QueryExecution.executedPlan(SQLContext.scala:265)
[info]   at org.apache.spark.sql.SQLContext$QueryExecution.toRdd$lzycompute(SQLContext.scala:268)
[info]   at org.apache.spark.sql.SQLContext$QueryExecution.toRdd(SQLContext.scala:268)
[info]   at org.apache.spark.sql.SchemaRDDLike$class.saveAsParquetFile(SchemaRDDLike.scala:66)
[info]   at org.apache.spark.sql.SchemaRDD.saveAsParquetFile(SchemaRDD.scala:98)
```