=act,rem #13946 fix handling of null parameters in Props

+ `Props` creation with `nulls` does not fail any longer
+ `null` values are now serialised "properly" and can be used in remote deployments too. `"null"` marker is being used instead of class, it's theoretically redundant, but may be useful for debugging
+ added tests for `Reflect.findConstructor`
- does not change wire protocol, so that's open for discussion (if we want it to be more like SerialisedMessage or not)

Resolves #13946