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.
so if i have a file named literally "README" or "readme" with no extension then this also fails and is rejected as well?

should probably add a spec test around this case.