#52 return std::make_optional<> in TextStorageString::convertToUnwrappedLineDescr()

[Alexey Dokuchaev]

This causes compilation errors with Clang of no matching constructor for initialization of 'optional<unwrappedlinedescr>' because various candidate templates are ignored or not viable; removing it fixes the build on FreeBSD. Do we need to call std::make_optional<> here at all? Notice that the similar TextStorageString::getRealLinePair() method above which also returns std::optional<> does it directly.