Disallow blank (non-null) values for a Note's `line_code` attribute
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.