Implementation of the partial PATCH update in API platform 3

[Yasix]

In API platform 2 there was a DataTransformers concept and an ability to create DataTransformer which took raw $data argument from the Request BEFORE the validation. It gave an ability to load missing Request fields from db entity.

Now at API Platform 3 it's recomended to use StateProcessors intead of DataProviders. BUT! $data argument of StateProcessor::__invoke is passed AFTER the validation. So if Request has some missing fields and those fields are NotBlank-constrained there validation exception raised and StateProcessor isn't called at all!

Please tell me how can i implement partial PATCH undr API plaform 3 ?
How can I load missing fields from db for incomplete PATH request?