Add unit specs for `Note#active?`

See merge request !3133

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.

These three tests were essentially checking that adding a note to
something updated its `noteable`'s `updated_at` attribute.

This is well-tested Rails behavior and we shouldn't feel the need to
write an integration test to verify it. At most we should be ensuring
that the association definition adds the `touch: true` option, which we
now do in Note's unit test.