Make zero-length READMEs behave like no README

:convenience_store: 

This will make things a lot more clear for cookbooks with a zero-length README, instead of showing opaque 500 errors.  In the future, we can look at possibly removing the requirement for a README entirely.
In my testing, the value being returned from `read.contents` was `nil`, but `blank?` certainly doesn't hurt anything.  Changed.