Symfony version(s) affected: 4.2.0

Description
4.2.0 attempts to call isInitializable() on my extractor that doesn't implement PropertyInitializableExtractorInterface.
Probable caused by the @dunglas changes in #26997

How to reproduce
I have a custom ReflectionExtractor service that decorates the default one:

MyNamespace\ReflectionExtractor:
    decorates: property_info.reflection_extractor

class ReflectionExtractor implements PropertyListExtractorInterface, PropertyTypeExtractorInterface, PropertyAccessExtractorInterface
{
}

It used to work before 4.2.0, but I now get an UndefinedMethodException because it doesn't have an isInitializable() method.

Possible Solution
I haven't checked further, but it seems to be a problem with the configuration: my extractor is injected in the $initializableExtractors constructor argument of PropertyInfoExtractor, even though it doesn't implement PropertyInitializableExtractorInterface.