Include empty VERSION_CODENAME= field in os-release
    
    /etc/os-release now supports a VERSION_CODENAME field that should
    contain the codename of a release if any exists. Fedora does not
    use a codename, so it should be set to the empty string.
    
    This is because some tools (such as https://github.com/nir0s/distro
    for Python) attempt to interpret first the VERSION_CODENAME and
    then subsequently fall back to parsing the VERSION field for a
    codename within parentheses. This means that the tools will return
    a different value of codename depending on which Edition was
    originall installed. Instead, we should ensure that it returns no
    codename at all.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>