title: Ruby canvas_cc gem is causing errors
body: <p>Attempting to use the Ruby 'canvas_cc' gem to create .imscc cartridges, but running into issues that I am having difficulty trouble shooting. I followed the basic documentation in the readme, but I get an issue from within the gem.</p>
  
  <p>Ruby</p>
  
  <pre><code>require 'tmpdir'
  course = CanvasCc::CanvasCC::Models::Course.new
  course.identifier = "cc"
  course.title =  "testing cc"
  dir = Dir.mktmpdir
  output_dir = CanvasCc::CanvasCC::CartridgeCreator.new(course).create(dir)
  </code></pre>
  
  <p>Output</p>
  
  <pre><code>Traceback (most recent call last):
          5: from course_creation.rb:25:in `&lt;main&gt;'
          4: from /Users/me/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/canvas_cc-0.0.43/lib/canvas_cc/canvas_cc/cartridge_creator.rb:18:in `create'
          3: from /Users/me/.rbenv/versions/2.5.1/lib/ruby/2.5.0/tmpdir.rb:89:in `mktmpdir'
          2: from /Users/me/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/canvas_cc-0.0.43/lib/canvas_cc/canvas_cc/cartridge_creator.rb:19:in `block in create'
          1: from /Users/me/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/canvas_cc-0.0.43/lib/canvas_cc/canvas_cc/cartridge_creator.rb:45:in `write_cartridge'
  /Users/me/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/canvas_cc-0.0.43/lib/canvas_cc/canvas_cc/grading_standard_writer.rb:11:in `write': undefined method `empty?' for nil:NilClass (NoMethodError)
  </code></pre>
  
  <p>Also tried </p>
  
  <pre><code>require 'canvas_cc'
  course = CanvasCc::CanvasCC::Models::Course.new
  course.identifier = "cc"
  course.title =  "testing cc"
  dir = Dir.mkdir "cartridge"
  output_dir = CanvasCc::CanvasCC::CartridgeCreator.new(course).create(Dir.pwd + "/cartridge")
  </code></pre>
  
  <p>I am getting the same error running the second script as the first. </p>
  
  <p>I've tried to understand what is wrong by looking through both the documentation and the code in the gem, but I can't seem to debug it. I am using Ruby 2.5.1 with rbenv, but I have tried multiple other ruby versions with the same issue.</p>
  
  <p>My goal is to just create basic common cartridges with this gem, but I can't even do that. Any help would be much appreciated with this!</p>