Disallow blank (non-null) values for a Note's `line_code` attributers-disallow-blank-line-code

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.

1 files changed, 9 insertions, 0 deletions

diff --git a/db/migrate/20160307221555_disallow_blank_line_code_on_note.rb b/db/migrate/20160307221555_disallow_blank_line_code_on_note.rb
new file mode 100644
index 00000000000..49e787d9a9a
--- /dev/null
+++ b/db/migrate/20160307221555_disallow_blank_line_code_on_note.rb
@@ -0,0 +1,9 @@
+class DisallowBlankLineCodeOnNote < ActiveRecord::Migration
+  def up
+    execute("UPDATE notes SET line_code = NULL WHERE line_code = ''")
+  end
+
+  def down
+    # noop
+  end
+end