support NULL on blob and char columns

[unconsolable]

Since #712 is merged, may NULL on blob and char columns can be supported.

Currently, NULL on these columns will become default value.

> create table test (e varchar(10));
created
in 0.031s
> insert into test values (NULL);
1 rows inserted
in 0.035s
> select * from test;
+---+
| e |
+---+
|   |
+---+
in 0.003s

[unconsolable]

I'm going to try to work on it. If anyone is working on it now, please let me know. If there are not any progress for a long time (e.g., two months), feel free to take over.