document: return nil if bad arg in #<=>
document: return nil if bad arg in #<=>

Addresses @envygeek's comment:

https://github.com/jekyll/jekyll/commit/233589e15076c41ac4b95ad6e0595f2f6a4a7ac2#commitcomment-15164178

floydpraveen/jekyll,darwin/jekyll,Strangehill/jekyll,zsyed91/jekyll,noikiy/jekyll,stomar/jekyll,x-way/jekyll,rlugojr/jekyll,noikiy/jekyll,chrisfinazzo/jekyll,floydpraveen/jekyll,brint/jekyll,mrb/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,rlugojr/jekyll,jekyll/jekyll,alfredxing/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,pmarsceill/jekyll,floydpraveen/jekyll,tomjohnson1492/jekyll,gencer/jekyll,tomjohnson1492/jekyll,alex-kovac/jekyll,stomar/jekyll,bsmr-ruby/jekyll,ryanshaw/jekyll,brint/jekyll,x-way/jekyll,gencer/jekyll,derekgottlieb/jekyll,mrb/jekyll,alfredxing/jekyll,tomjohnson1492/jekyll,tillgrallert/jekyll,brint/jekyll,jaybe-jekyll/jekyll,gencer/jekyll,noikiy/jekyll,Strangehill/jekyll,rasa2011/rasa2011.github.io,princeofdarkness76/jekyll,nasht00/jekyll,Strangehill/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,ajhit406/jekyll,darwin/jekyll,x-way/jekyll,xcatliu/jekyllcn,Kinghack/kinghack.github.com,Strangehill/jekyll,ryanshaw/jekyll,tomjohnson1492/jekyll,alfredxing/jekyll,tomjohnson1492/jekyll,alex-kovac/jekyll,ryanshaw/jekyll,darwin/jekyll,zsyed91/jekyll,floydpraveen/jekyll,tillgrallert/jekyll,mrb/jekyll,rasa2011/rasa2011.github.io,zsyed91/jekyll,Kinghack/kinghack.github.com,fulldecent/jekyll,gencer/jekyll,alfredxing/jekyll,nasht00/jekyll,noikiy/jekyll,jeffkole/jekyll,xiebinhqy/jklly_china,pmarsceill/jekyll,princeofdarkness76/jekyll,tillgrallert/jekyll,jeffkole/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,ZDroid/jekyll,nasht00/jekyll,getandpost/jekyll.github.io,rlugojr/jekyll,xiebinhqy/jklly_china,xcatliu/jekyllcn,bsmr-ruby/jekyll,gencer/jekyll,alex-kovac/jekyll,jeffkole/jekyll,zsyed91/jekyll,princeofdarkness76/jekyll,stomar/jekyll,jaybe-jekyll/jekyll,princeofdarkness76/jekyll,xiebinhqy/jklly_china,ZDroid/jekyll,stomar/jekyll,mrb/jekyll,fulldecent/jekyll,derekgottlieb/jekyll,brint/jekyll,rlugojr/jekyll,xcatliu/jekyllcn,ajhit406/jekyll,yhironaka/yhironaka.github.io,alex-kovac/jekyll,x-way/jekyll,Strangehill/jekyll,fulldecent/jekyll,nasht00/jekyll,ZDroid/jekyll,xiebinhqy/jklly_china,jekyll/jekyll,darwin/jekyll,rasa2011/rasa2011.github.io,xcatliu/jekyllcn,chrisfinazzo/jekyll,jeffkole/jekyll,getandpost/jekyll.github.io,bsmr-ruby/jekyll,pmarsceill/jekyll,Kinghack/kinghack.github.com,bsmr-ruby/jekyll,jaybe-jekyll/jekyll,derekgottlieb/jekyll,ajhit406/jekyll,jekyll/jekyll,pmarsceill/jekyll,ajhit406/jekyll,derekgottlieb/jekyll,alex-kovac/jekyll