PHP warnings on BlockDisplayVariant::setConfiguration()

This is a quick follow-up to #2639526: Calling BlockDisplayVariant::setConfiguration() won't change results returned from plugin collections

The code that was committed works great if your calling BlockDisplayVariant::setConfiguration($config) with a config array that you got from calling BlockDisplayVariant::getConfiguartion() - however, it's actually quite common to call setConfiguration([]) when creating a new variant! In that case, there are some (harmless) PHP warnings.

Basically, we need to use the configuration after the defaults are mixed in to prevent these warnings. I'll attach a patch in a moment.