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.
"must contain a non-empty README.md or other README file with an extension"

(which is still kind of a terrible error message...)