Migrations shouldn't fail RuboCop checks - especially lint checks, such
as the nested method check. To avoid changing code in existing
migrations, add the magic comment to the top of each of them to skip
that file.
It's unclear how these blank values got added, but GitLab.com had a few:
```
irb(main):002:0> Note.where("line_code IS NOT NULL AND line_code = ''").count
=> 439
```
We've added a migration to convert any existing records to use a NULL
value when blank, and updated Note to set blank values to nil before
validation.
