tests: avoid empty optional::emplace() on IndirectDescriptor (fixes Linux build)
clang with libstdc++ 14 does not treat std::optional<DescriptorSource::IndirectDescriptor>
as default-constructible (nested struct with default member initializers), so the
argument-less emplace() fails to resolve. Pass an explicit value-initialized
IndirectDescriptor, matching the form already used elsewhere in this file.
