Allow to provide a default value to getValue function

[ppalomera]

Instead of returning an empty array, we should have the option to define a default value to return if our config is empty. Like is done in drupal state and drupal settings:

public function getValue($siteKey, $field, $defaultValue = []) {}

[jherencia]

I do not see why [] is a valid $defaultValue, it should be NULL instead, right?

[ppalomera]

I do not see why [] is a valid $defaultValue, it should be NULL instead, right?

Yes, I assumed [] as default value to be compliant with current solution, but I have changed it to NULL