FIX: verify presence of 'sso url' before enabling 'enable sso'

lib/validators/enable_sso_validator.rb

@@ -0,0 +1,14 @@
+class EnableSsoValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    return true if val == 'f'
+    SiteSetting.sso_url.present?
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.sso_url_is_empty')
+  end
+end