[SPARK-4077][SQL] Spark SQL return wrong values for valid string timestamp values

In org.apache.hadoop.hive.serde2.io.TimestampWritable.set , if the next entry is null then current time stamp object is being reset. 
However because of this hiveinspectors:unwrap cannot use the same timestamp object without creating a copy.

Author: Venkata Ramana G <ramana.gollamudi@huawei.com>