fix: migrate column default value

[a631807682]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

relate to #5325 and go-gorm/mysql#75

mariadb test will pass after go-gorm/mysql#75 merged

User Case Description

[sc-ms]

Hey, I saw your fix but in my case I have for
dv, dvNotNull := columnType.DefaultValue()
these values:
dv: "NULL"
dvNotNull: true

So it has alterColumn = true but in migrator it doesn't change the column.

[a631807682]

@sc-ms can you provide an example?

[sc-ms]

I found the reason why it didn't work for me.
I noticed it while creating an example.
I had still used gorm.io/driver/mysql in v1.3.3.
But with v1.3.4 it works correctly.

Please excuse me for the confusion.