Avoid depending on argument default constructors #41

CTransaction default constructor was removed in
faac31521bb7ecbf999541cf918d3750ff589de4
https://github.com/bitcoin/bitcoin/pull/20588 which exposed two places
where libmultiprocess was inadvertently assuming default constructors
were defined.

In both places, it would never actually call the default constructor
at runtime, just rely on it being present during compile time code
generation.

In the first case, compiler would fail while trying to figure out the type of
`std::invoke_result_t<EmplaceFn>` where EmplaceFn called
`make_shared<CTransaction>()`. This was fixed by just the moving expression
into an unexpanded template method.

In the second case code was calling Optional<CTransaction>::emplace() to
default-initialize CTransaction& output-arguments. It was fixed by
moving into a constexpr-if that would be known to be false for output
arguments at compile time.