don't check for avatar service enabled unless needed

improves performance on user_json

closes #CNVS-25270

Change-Id: Ib18d8570dabb3aa7f50571280ab93c269b0adb21
Reviewed-on: https://gerrit.instructure.com/67691
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

lib/api/v1/user.rb

@@ -60,7 +60,7 @@ module Api::V1::User
           json[:login_id] = pseudonym.unique_id
         end
       end
-      if user.account.service_enabled?(:avatars) && includes.include?('avatar_url')
+      if includes.include?('avatar_url') && user.account.service_enabled?(:avatars)
         json[:avatar_url] = avatar_url_for_user(user, blank_fallback)
       end
       if enrollments