In XDG Base Directory Specification what if $HOME is unset or empty

Personally I'd like to adhere to to XDG Base Directory Specification to make my utility more portable. However when doing so it requires me to re-implement for systems that do not have the XDG_ parameters implemented. This on it's own is not much of a problem as the XDG Base Directory Specification covers this detailing what to do instead. For example if XDG_DATA_HOME is unset or empty use ~/.local/share instead. However what to do if $HOME is unset or empty? Should I adhere to the system wide options at least for where it is in the XDGBDS given (e.g. no $HOME but more /etc/*something* etc.)? Honestly I have very little experience under which circumstances $HOME actually is empty, so it's hard for me to decide well here.