Ensure `LotusJson<Option<T>>` is marked as `optional` in the OpenRPC spec

[lemmih]

Does this fix an encountered issue? Perhaps a simple test is in order to verify that LotusJson<Option<T>> encodes/decodes the way we expect. It's a couple of lines of code, but goes a long way, especially if this code is ever touched by somebody else.

I leave this up to you, non-blocking.

Originally posted by @ruseinov in https://github.com/ChainSafe/forest/issues/4330#issuecomment-2113427560