=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
+ 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
Merge pull request #15782 from ktoso/wip-props-with-nulls-ktoso

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