find_by_ref should expect empty array if list empty

Some folks on my team were getting this error:

https://gist.github.com/khristian/6718700

When trying to do 'zenflow feature review'. Our repo doesn't have any
open pull requests, and in that case 'list' returns [] not nil, which is
what find_by_ref seems to expect.

As a result, when list returned [], find_by_ref would ignore the line if
!list.nil? and try to proceed to call detect on list which inside the
block resulted in the error I posted from the gist above.
Update to a positive assertion
Merge pull request #11 from jondkinney/list_returns_empty_array_not_nil

List returns empty array not nil when no pull requests are present on a repo.