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
Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones. For example:
#include <cm/string_view>
can be used safely for CMake development in place of the `<string_view>`
standard header.
Fixes: #19491
Sean McBride