fix binary value null value problem

[lichunzhu]

What problem does this PR solve?

Dumpling doesn't handle null value for binary type variable.

What is changed and how it works?

Handle null value when dumping binary columns.

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

Release note

• Handle null value when dumping binary columns.

[kennytm]

could you add a test case for

create table a(c blob);
insert into a values (x'');
insert into a values (null);

rest LGTM

[kennytm]

lgtm