Fix restricted visibility bugs

Check for nil values in the restricted_visibility_level validation
method, and set the restricted visibility request parameter to `[]` when
it's missing from the request.

app/controllers/admin/application_settings_controller.rb

@@ -21,7 +21,9 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
 
   def application_setting_params
     restricted_levels = params[:application_setting][:restricted_visibility_levels]
-    unless restricted_levels.nil?
+    if restricted_levels.nil?
+      params[:application_setting][:restricted_visibility_levels] = []
+    else
       restricted_levels.map! do |level|
         level.to_i
       end

app/models/application_setting.rb

@@ -27,12 +27,14 @@ class ApplicationSetting < ActiveRecord::Base
     if: :home_page_url_column_exist
 
   validates_each :restricted_visibility_levels do |record, attr, value|
+    unless value.nil?
       value.each do |level|
         unless Gitlab::VisibilityLevel.options.has_value?(level)
           record.errors.add(attr, "'#{level}' is not a valid visibility level")
         end
       end
     end
+  end
 
   def self.current
     ApplicationSetting.last