Updated: Comment #N
At the moment we have to do things like
$this->state->get('drupal_css_cache_files') ?: array(); and
$this->state->get('language_count') ?: 1 because we can;t return a default value if no key is found.
This is what we could do with variable_get() calls before, Settings has it, and with symfony ParameterBags you can also do the same.
Add a $default parameter to KeyValueStoreInterface::get() method. This will allow us to do things like
$this->state->get('drupal_css_cache_files', array()) and
Patch, review, etc..
User interface changes
Optional $default parameter on state->get() calls for example.