Fixes #255

The parsing code (get_repos_in_vcstool_format()) already provides a useful error message when an attribute is missing (e.g., url:). However, as shown in the issue above, it doesn't handle/detect empty repository entries. Therefore, check if the repository is empty before trying to access its attributes. Otherwise, provide a useful error message:

$ cat bad.repos
repositories:
  empty_entry:
  missing_url:
    type: git
$ vcs validate --input bad.repos 
Repository 'empty_entry' is empty
Repository 'missing_url' does not provide the necessary information: 'url'

Before this change, validation fails on empty_entry (and does not validate missing_url):

$ vcs validate --input bad.repos 
Input data is not valid format: 'NoneType' object is not subscriptable

I've also updated the tests to cover this.