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.
Not sure it's necessary to set `readme` to `nil` -- it doesn't hurt, but it shouldn't be necessary, as evidenced by the other ["missing_readme" error condition](https://github.com/opscode/supermarket/blob/zero-length-readme/app/models/cookbook_upload/parameters.rb#L204).