include: fix 'no implicit conversion of nil to String'
include: fix 'no implicit conversion of nil to String'

This is when either 'dir' or 'file' is nil.

alex-kovac/jekyll,tomjohnson1492/jekyll,fulldecent/jekyll,gencer/jekyll,rlugojr/jekyll,ryanshaw/jekyll,Strangehill/jekyll,yhironaka/yhironaka.github.io,x-way/jekyll,fulldecent/jekyll,Strangehill/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,rasa2011/rasa2011.github.io,getandpost/jekyll.github.io,getandpost/jekyll.github.io,alex-kovac/jekyll,tomjohnson1492/jekyll,rlugojr/jekyll,jekyll/jekyll,tomjohnson1492/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,rasa2011/rasa2011.github.io,gencer/jekyll,Strangehill/jekyll,jekyll/jekyll,xiebinhqy/jklly_china,gencer/jekyll,Strangehill/jekyll,jaybe-jekyll/jekyll,rlugojr/jekyll,rasa2011/rasa2011.github.io,ryanshaw/jekyll,xiebinhqy/jklly_china,x-way/jekyll,alex-kovac/jekyll,fulldecent/jekyll,xiebinhqy/jklly_china,xiebinhqy/jklly_china,tillgrallert/jekyll,getandpost/jekyll.github.io,fulldecent/jekyll,chrisfinazzo/jekyll,tillgrallert/jekyll,gencer/jekyll,tomjohnson1492/jekyll,jaybe-jekyll/jekyll,chrisfinazzo/jekyll,chrisfinazzo/jekyll,tomjohnson1492/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,jekyll/jekyll,tillgrallert/jekyll,x-way/jekyll,rlugojr/jekyll,alex-kovac/jekyll,gencer/jekyll,alex-kovac/jekyll,x-way/jekyll,Strangehill/jekyll,chrisfinazzo/jekyll,jekyll/jekyll