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.
Agreed, I'll remove the validation on the extension and add another spec to ensure that everything works with just a straight "README" file with no extension.