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.
Thanks for the suggestions.  I was struggling with a good message there :grin: I'll remove the validation on the extension, so we don't need to specify that.