Enable Gaudi::Functional::Transformer<Result(const EventContext&)>

This variant was not tested, and fell through the cracks, as it ends up with 'no input datahandles', which requires a special case in Transformer::execute. And as 'no input datahanles' is also exactly what used to distinguish a Producer from a Transformer, define Producer as nothing more than a Transformer without inputs...

To simplify the support for this case, also change the 'empty' data handle recognition in DataHandleMixin by using std::tuple<> instead of void to flag the absence of handles...