[utils] Add fromCstring() that safely converts NULL to empty string.
[utils] Add fromCstring() that safely converts NULL to empty string.

rpm-software-management/libdnf,Conan-Kudo/libhif,hughsie/libhif,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,rpm-software-management/libhif,hughsie/libhif,Conan-Kudo/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libhif,Conan-Kudo/libhif