[FCHost][Linux] 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)
