=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
I don't know if we have a decision on what import style to use. My only preference is to use something that works with the tooling, including something that doesn't change in different ways depending on who touched the file. Eclipse works better with one import per line, and I think that generates less merge conflicts - so that is my preference. WDYT?