Function optionalCustom

This is the idiomatic usage pattern for optional(custom(...)).

optional only affects serialization so we must make sure to check if the value is undefined or not, if yes, serialize, else, return undefined which will be processed by optional(...) as SKIP.

optional does not affect deserialization, however, as undefined values are automatically skipped See https://github.com/mobxjs/serializr/issues/73#issuecomment-535641545