Prevent crash constructing string from nulltpr

If XDG_CURRENT_DESKTOP isn't set, getenv returns a `nullptr`.
Constructing a `std::string` from `nullptr` will throw an exception.
This prevents a crash when not using a DE, but zenity or kdialog is
installed regardless

(also fixes a typo where the fallthrough attribute wasn't applied to
anything due to a missing semicolon)