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

[zsxwing]

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.

[SparkQA]

QA tests have started for PR 2856 at commit 51da655.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2856 at commit 51da655.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/21915/
Test PASSed.

[aarondav]

LGTM, merging into master - thanks!