## bug for all version in file type_info.cpp
`TIMESTAMP NULL` is missing null flag
```C++
    sql_type_info("TIMESTAMP NOT NULL", typeid(sql_timestamp),
            MYSQL_TYPE_TIMESTAMP),
    ...
    sql_type_info("TIMESTAMP NULL", typeid(Null<sql_timestamp>),
            MYSQL_TYPE_TIMESTAMP),
```

## patch
```C++
diff -ruN
--- 2020-02-28 04:31:27.000000000 +0800
+++ 2020-07-17 12:34:29.700736347 +0800
@@ -178,7 +178,7 @@
        sql_type_info("NULL NULL", typeid(Null<void>),
                        MYSQL_TYPE_NULL, mysql_ti_sql_type_info::tf_null),
        sql_type_info("TIMESTAMP NULL", typeid(Null<sql_timestamp>),
-                       MYSQL_TYPE_TIMESTAMP),
+                       MYSQL_TYPE_TIMESTAMP, mysql_ti_sql_type_info::tf_null),
        sql_type_info("BIGINT NULL", typeid(Null<sql_bigint>),
                        MYSQL_TYPE_LONGLONG, mysql_ti_sql_type_info::tf_default |
                        mysql_ti_sql_type_info::tf_null),
```