customer and userId can be null when directly visiting account/profile and therefore trigger a log entry

[ellogwen]

When a bot/user directly visits the /account/profile controller-action and is not logged in, than you will get an entry in your server error log due to an exception thrown in Subscriber/PluginFrontendSubscriper.php because the method is missing checks that userId or customer are not empty

[ellogwen]

i created a PR #14