Merge branch 'fix/application-settings-error-default-value' into 'master'

Fix Error 500 when accessing application settings due to nil disabled OAuth sign-in sources

## What does this MR do?

When upgrading to GitLab 8.8, an admin would encounter an Error 500 due to a `nil` `disabled_oauth_sign_in_sources`. This MR ensures that the value will be a blank array if empty.

Closes #17564

See merge request !4242

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
+v 8.8.2 (unreleased)
+  - Fix Error 500 when accessing application settings due to nil disabled OAuth sign-in sources
+
 v 8.8.1
   - Add documentation for the "Health Check" feature
   - Allow anonymous users to access a public project's pipelines

app/models/application_setting.rb

@@ -7,7 +7,7 @@ class ApplicationSetting < ActiveRecord::Base
 
   serialize :restricted_visibility_levels
   serialize :import_sources
-  serialize :disabled_oauth_sign_in_sources
+  serialize :disabled_oauth_sign_in_sources, Array
   serialize :restricted_signup_domains, Array
   attr_accessor :restricted_signup_domains_raw