This introduces the minimal changes for FunTuple to understand the optional return type of Functors::Optional<T>
Note that Rec!2977 (merged) does not yet change the behavior of e.g. MapRelInputToFunOuput. These Functors still return their constexpr invalid_value.
My proposal would be to merge Rec!2977 (merged) as is + this MR.
Then I can create a follow-up MR that removes the current usage of the constexpr invalid_value member to replace it with the new optional return type functionality.
But for that we also need to adapt FunTuple with the features to let us specify a default invalid value for a specific column in the tuple.
@amathad I am hoping you might have time to help with the latter? I already looked at FunTuple.cpp but wasn't sure where this should best be implemented.
