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.

app/models/note.rb

@@ -44,6 +44,7 @@ class Note < ActiveRecord::Base
   delegate :name, :email, to: :author, prefix: true
 
   before_validation :set_award!
+  before_validation :clear_blank_line_code!
 
   validates :note, :project, presence: true
   validates :note, uniqueness: { scope: [:author, :noteable_type, :noteable_id] }, if: ->(n) { n.is_award }
@@ -63,7 +64,7 @@ class Note < ActiveRecord::Base
   scope :nonawards, ->{ where(is_award: false) }
   scope :for_commit_id, ->(commit_id) { where(noteable_type: "Commit", commit_id: commit_id) }
   scope :inline, ->{ where("line_code IS NOT NULL") }
-  scope :not_inline, ->{ where(line_code: [nil, '']) }
+  scope :not_inline, ->{ where(line_code: nil) }
   scope :system, ->{ where(system: true) }
   scope :user, ->{ where(system: false) }
   scope :common, ->{ where(noteable_type: ["", nil]) }
@@ -375,6 +376,10 @@ class Note < ActiveRecord::Base
 
   private
 
+  def clear_blank_line_code!
+    self.line_code = nil if self.line_code.blank?
+  end
+
   def awards_supported?
     (for_issue? || for_merge_request?) && !for_diff_line?
   end

db/migrate/20160307221555_disallow_blank_line_code_on_note.rb

+class DisallowBlankLineCodeOnNote < ActiveRecord::Migration
+  def up
+    execute("UPDATE notes SET line_code = NULL WHERE line_code = ''")
+  end
+
+  def down
+    # noop
+  end
+end

spec/models/note_spec.rb

@@ -226,4 +226,12 @@ describe Note, models: true do
       expect(note.is_award?).to be_falsy
     end
   end
+
+  describe 'clear_blank_line_code!' do
+    it 'clears a blank line code before validation' do
+      note = build(:note, line_code: ' ')
+
+      expect { note.valid? }.to change(note, :line_code).to(nil)
+    end
+  end
 end