Changes filename parameter of AliasFiles to empty string instead of nil

This changes the filename parameter of AliasFiles created during generation to empty string ('') instead of `nil`.

Some Jekyll plugins examine the filename of each StaticFile in `site.static_files` to determine what filetype to operate on-- one example that broke for me was https://github.com/zroger/jekyll-less (specifically, https://github.com/zroger/jekyll-less/blob/master/lib/jekyll-less.rb#L57).

If the filename is `nil`, you get this error message:

```
/Users/michael/.rvm/gems/ruby-2.0.0-p195/gems/jekyll-1.0.3/lib/jekyll/static_file.rb:21:in `join': no implicit conversion of nil into String (TypeError)
```

Unless there's a good reason for it to be `nil`, changing it to empty string fixes this.