Check if var exists before include tag (#7530)
Check if var exists before include tag (#7530)

Merge pull request 7530
fulldecent/jekyll,tillgrallert/jekyll,Strangehill/jekyll,Strangehill/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,yhironaka/yhironaka.github.io,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,x-way/jekyll,jekyll/jekyll,x-way/jekyll,jekyll/jekyll,fulldecent/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,Strangehill/jekyll,fulldecent/jekyll,Strangehill/jekyll,chrisfinazzo/jekyll,ryanshaw/jekyll,x-way/jekyll,Strangehill/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,x-way/jekyll,tillgrallert/jekyll,tillgrallert/jekyll