[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.gollamudihuawei.com>

Author: Venkata Ramana Gollamudi <ramana.gollamudi@huawei.com>

Closes #3019 from gvramana/spark_4077 and squashes the following commits:

32d818f [Venkata Ramana Gollamudi] fixed check style
fa01e71 [Venkata Ramana Gollamudi] cloned timestamp object as org.apache.hadoop.hive.serde2.io.TimestampWritable.set will reset current time object