Minor change to document.rb to prevent exception if sibling is nil

[tmpfs]

Nothing major, just came across a situation where sibling was nil (with valid ronn/markdown) and an exception was thrown.

Maybe on your next pass you could merge this one too?

Cheers!

[tmpfs]

FYI, tests passing with the exceptions noted on the previous pull request.

[rtomayko]

:cool: Merging. Thanks!