Add a Pro Tip to docs about front matter variables being optional

Added a note to Front Matter in the docs about empty YAML.

I use (and have seen other people use) `layout=nil` to get jekyll to process things that aren't pages, most commonly atom.xml, but after testing it just now in 1.0, the user can just leave the YAML empty and still get Liquid processing...

Thanks! :smile: 
MM