Tests: Fix reliance on undefined behavior of cm::optional

The test for cm::optional assumed that *opt would return a reference to a non-constructed object if opt.has_value() is false. However, on certain systems that build CMake with flags to harden the build, this condition aborts rather than returning the invalid reference. Fix this by getting the reference only when the cm::optional is known to actually have a value.

Fixes: #19964 (closed)

Backport: release