Allow optional properties with `RequestToObjectMapper`

[gglnx]

Description

I've got an PATCH endpoint that allows partial updates to an object. Some properties are nullable, so I can't use null to detect if a change is wished. Setting no default on the target properties throws validation error (property must be specified). I've would like mark properties as optional and leave them unset if they are missing in the request and only get validated if they in the request.

[brendt]

Can you share some code?

[gglnx]

Yes: https://github.com/gglnx/demo-tempest-post-patch

I hope the repo explains what I mean. The CreatePost should be fully validated and flagged for missing values. Properties in UpdatePost should be skipped if missing in the request body. Using default null would overwritten expiryDate if not also provided in the PATCH request.

[gglnx]

So marking properties with #[SkipIfMissing] or adding onlyIfMissing: true to #[SkipValidation] would be nice.