[SPARK-4008] Fix "kryo with fold" in KryoSerializerSuite

`zeroValue` will be serialized by `spark.closure.serializer` but `spark.closure.serializer` only supports the default Java serializer. So it must not be `ClassWithoutNoArgConstructor`, which can not be serialized by the Java serializer.

This PR changed `zeroValue` to null and updated the test to make it work correctly.

Author: zsxwing <zsxwing@gmail.com>

Closes #2856 from zsxwing/SPARK-4008 and squashes the following commits:

51da655 [zsxwing] [SPARK-4008] Fix "kryo with fold" in KryoSerializerSuite