Ensure node[:tags] is not equal to nil

Started using tagging recently and noticed that tagged? and the like fail on nodes where node[:tags] is equal to nil. This PR ensures that node[:tags] defaults to Array.
can you use `if !attribute?(:tags) || node[:tags].nil?` instead?

this is one of the rare occasions that i agree with the ruby style guide:

https://github.com/bbatsov/ruby-style-guide#no-and-or-or

don't want to have to stop and think about the implications of lower precedence on the boolean operators and recall where the lower precedence operators will troll you, and just want to have them have higher precedence.