Fix fedora nginx::sources recipe error

Error when running nginx::sources recipe on Fedora:

```
  ================================================================================
  Recipe Compile Error in /root/.chef/local-mode-cache/cache/cookbooks/nginx/recipes/source.rb
  ================================================================================
  
  NoMethodError
  -------------
  undefined method `empty?' for nil:NilClass
  
  Cookbook Trace:
  ---------------
    /root/.chef/local-mode-cache/cache/cookbooks/nginx/recipes/passenger.rb:25:in `from_file'
    /root/.chef/local-mode-cache/cache/cookbooks/nginx/recipes/source.rb:91:in `block in from_file'
    /root/.chef/local-mode-cache/cache/cookbooks/nginx/recipes/source.rb:90:in `each'
    /root/.chef/local-mode-cache/cache/cookbooks/nginx/recipes/source.rb:90:in `from_file'
  
  Relevant File Content:
  ----------------------
  /root/.chef/local-mode-cache/cache/cookbooks/nginx/recipes/passenger.rb:
  
   18:  #
   19:  
   20:  packages = value_for_platform_family(
   21:    %w(rhel)   => node['nginx']['passenger']['packages']['rhel'],
   22:    %w(debian) => node['nginx']['passenger']['packages']['debian']
   23:  )
   24:  
   25>> unless packages.empty?
   26:    packages.each do |name|
   27:      package name
   28:    end
   29:  end
   30:  
   31:  gem_package 'rake'
   32:  
   33:  gem_package 'passenger' do
   34:    action     :install
```