Fix bug where #asset_path could raise TypeError

It raised ```#<TypeError: no implicit conversion of nil into String>``` from ```Pathname#new``` when passed a blank ```pathname```.  ```AssetNotFound``` error was expected in this case too.

Such a request is actually made by [bootstrap-sass](https://github.com/twbs/bootstrap-sass/blob/master/vendor/assets/stylesheets/bootstrap/_variables.scss) via ```twbs-font-path()``` function.
```sass
// a flag to toggle asset pipeline / compass integration
// defaults to true if twbs-font-path function is present (no function => twbs-font-path('') parsed as string == right side)
// in Sass 3.3 this can be improved with: function-exists(twbs-font-path)
$bootstrap-sass-asset-helper: (twbs-font-path("") != unquote('twbs-font-path("")')) !default;
```