Find a safer way to call QUtf8Functions::fromUtf8()

Reference: https://codereview.qt-project.org/c/qt/qtbase/+/630171/comment/5737fe58_8f5310fe/

Coverity complains that we pass the address of a char32_t "singleton" to this function, which treats it as an array.

Discuss and implement an alternative (e.g. overload that returns a optional<>? char32_t)