Fix the case where file_name is nil.

This can happen on line 96, causing line 106 to blow up:


    /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bundler/gems/spin-110b19cdd888/lib/spin.rb:112:in `reject': undefined method `empty?' for nil:NilClass (NoMethodError)
      from /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bundler/gems/spin-110b19cdd888/lib/spin.rb:112:in `convert_push_arguments_to_files'
      from /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bundler/gems/spin-110b19cdd888/lib/spin.rb:41:in `push'
      from /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bundler/gems/spin-110b19cdd888/lib/spin/cli.rb:36:in `run'
      from /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bundler/gems/spin-110b19cdd888/bin/spin:3:in `<top (required)>'
      from /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bin/spin:23:in `load'
      from /Users/tyson/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/bin/spin:23:in `<main>'