net-im/psi: try to unbreak the build against Clang 21.1.8

Casting integers to enums outside of the valid range of the
enum is UB in C++17 and apparently no longer tolerated by
modern Clang even for C++11 code. Replace invalidAlgo (255)
with optional<T>-based approach using the C++11-compatible
Sy Brand's template implementation; std::optional is not an
option (pun intended) in this case because this code won't
compile in C++17 mode which it requires.

Obtained from: upstream (psi-im/iris commit 0d35992)
Reported by: pkg-fallout