Fixed 500 error when creating issue for project with no commits.

The form was updated to check that the repository wasn't empty before trying
to load the contribution guide.

This fixes #153.

app/views/projects/issues/_form.html.haml

 %div.issue-form-holder
   %h3.page-title= @issue.new_record? ? "New Issue" : "Edit Issue ##{@issue.iid}"
   %hr
-  - if @repository.contribution_guide && !@issue.persisted?
+  - if !@repository.empty? && @repository.contribution_guide && !@issue.persisted?
     - contribution_guide_url = project_blob_path(@project, tree_join(@repository.root_ref, @repository.contribution_guide.name))
     .alert.alert-info.col-sm-10.col-sm-offset-2
       ="Please review the <strong>#{link_to "guidelines for contribution", contribution_guide_url}</strong> to this repository.".html_safe