Remove empty Github service templates from db

When creating a new Github integration, it looks for a service
record of the same type, but with the template flag set to true,
and clone its attributes before presenting the record to the user.

`https://gitlab.com/gitlab-org/gitlab/merge_requests/15737/diffs`
added `static_context` as a default attribute on for
this service but it clashes with the empty template on .com.

This commit removes Github templates that have `nil` or `{}`
as properties, allowing new services to set the `static_context`
default value from the code.

changelogs/unreleased/remove-empty-github-service-templates.yml

+---
+title: Remove empty Github service templates from database
+merge_request: 18868
+author:
+type: fixed

db/post_migrate/20191021101942_remove_empty_github_service_templates.rb

+# frozen_string_literal: true
+
+## It's expected to delete one record on GitLab.com
+#
+class RemoveEmptyGithubServiceTemplates < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+
+  class Service < ActiveRecord::Base
+    self.table_name = 'services'
+    self.inheritance_column = :_type_disabled
+
+    serialize :properties, JSON
+  end
+
+  def up
+    relationship.where(properties: {}).delete_all
+  end
+
+  def down
+    relationship.find_or_create_by!(properties: {})
+  end
+
+  private
+
+  def relationship
+    RemoveEmptyGithubServiceTemplates::Service.where(template: true, type: 'GithubService')
+  end
+end

spec/migrations/remove_empty_github_service_templates_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20191021101942_remove_empty_github_service_templates.rb')
+
+describe RemoveEmptyGithubServiceTemplates, :migration do
+  subject(:migration) { described_class.new }
+
+  let(:services) do
+    table(:services).tap do |klass|
+      klass.class_eval do
+        serialize :properties, JSON
+      end
+    end
+  end
+
+  before do
+    services.delete_all
+
+    create_service(properties: nil)
+    create_service(properties: {})
+    create_service(properties: { some: :value })
+    create_service(properties: {}, template: false)
+    create_service(properties: {}, type: 'SomeType')
+  end
+
+  def all_service_properties
+    services.where(template: true, type: 'GithubService').pluck(:properties)
+  end
+
+  it 'correctly migrates up and down service templates' do
+    reversible_migration do |migration|
+      migration.before -> do
+        expect(services.count).to eq(5)
+
+        expect(all_service_properties)
+          .to match(a_collection_containing_exactly(nil, {}, { 'some' => 'value' }))
+      end
+
+      migration.after -> do
+        expect(services.count).to eq(4)
+
+        expect(all_service_properties)
+          .to match(a_collection_containing_exactly(nil, { 'some' => 'value' }))
+      end
+    end
+  end
+
+  def create_service(params)
+    data = { template: true, type: 'GithubService' }
+    data.merge!(params)
+
+    services.create!(data)
+  end
+end