Added a flash error message when an empty yaml is being imported

https://bugzilla.redhat.com/show_bug.cgi?id=1116704
```ruby
if params[:upload] && params[:upload][:file] && File.size(params[:upload][:file].tempfile) == 0
```
can be shortened to
```ruby
if params.fetch_path(:upload, :file) && File.size(params[:upload][:file].tempfile) == 0
```