PojoSerializerSnapshot is using incorrect ExecutionConfig when restoring serializer

org.apache.flink.api.java.typeutils.runtime.PojoSerializerSnapshot#restoreSerializer is using freshly created new ExecutionConfig() execution config for the restored serializer, which overrides any configuration choices made by the user. Most likely this is a dormant bug, since restored serializer shouldn't be used for serializing any new data, and the ExecutionConfig is only used for subclasses serializations that haven't been cached. If this is indeed the case, I would propose to change it's value to null and safeguard accesses to that field with an IllegalStateException.