Update pfsDesign/pfsConfig constructor to allow GuideStars=None

DAMD-101 introduced support for shuffle-supplied guide star information, and a corresponding GuideStars object needs to be passed to the PfsDesign or PfsConfig constructor.

There are cases where the guide star information is irrelevant. Presently in such cases, the user must still pass a GuideStars object (the value of GuideStars.empty()).

Please make this easier for the user by allowing the user to pass a value of None to the constructor, which will internally handle the creation of the corresponding empty GuideStars object.