[SPARK-19726][SQL] Faild to insert null timestamp value to mysql using spark jdbc

## What changes were proposed in this pull request?

when creating table like following:
> create table timestamp_test(id int(11), time_stamp timestamp not null default current_timestamp);

The result of Excuting "insert into timestamp_test values (111, null)" is different between Spark and JDBC.
```
mysql> select * from timestamp_test;
+------+---------------------+
| id   | time_stamp          |
+------+---------------------+
|  111 | 1970-01-01 00:00:00 | -> spark
|  111 | 2017-06-27 19:32:38 | -> mysql
+------+---------------------+
2 rows in set (0.00 sec)
```
   Because in such case ```StructField.nullable``` is false, so the generated codes of ```InvokeLike``` and ```BoundReference``` don't check whether the field is null or not. Instead, they directly use ```CodegenContext.INPUT_ROW.getLong(1)```, however, ```UnsafeRow.setNullAt(1)``` will put 0 in the underlying memory.

   The PR will ```always``` set ```StructField.nullable```  true after obtaining metadata from jdbc connection, Since we can insert null to not null timestamp column in MySQL. In this way, spark will propagate null to underlying DB engine, and let DB to choose how to process NULL.

## How was this patch tested?

Added tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: YIHAODIAN\wangshuangshuang <wangshuangshuang@yihaodian.com>
Author: Shuangshuang Wang <wsszone@gmail.com>

Closes #18445 from shuangshuangwang/SPARK-19726.