fixed Site#archives to no longer break Site#index
fixed Site#archives to no longer break Site#index

When Site#index calls Site#archives, it does so with a nil filter which
became // by default.  This made the expression `filter =~ /regex/`
bomb out spectacularly with a TypeError.  This change ensures that
filter is an empty string rather than an empty regexp when not supplied.

sbsurf/toto,wulftone/toto,cloudhead/toto,mattvanhorn/toto,nviarnes/customized_toto,mattvanhorn/toto,rschmukler/Glinda,mclaughj/toto-tags,ixti/toto,wulftone/toto,blanchma/toto-extended,stevepaulo/toto,D-Roch/Blog-toto,stevepaulo/toto,nviarnes/custom_toto