title: AttributeError: 'NoneType' object has no attribute 'decode'
body: <pre><code>names, plot_dicts = [], []
  for repo_dict in repo_dicts:
      names.append(repo_dict['name'])
      plot_dict = {
          'value': repo_dict['stargazers_count'],
          'label': repo_dict['description'],
          'xlink': repo_dict['owner']['html_url'],
          }
      plot_dicts.append(plot_dict)
  
  my_style = LS('#333366', base_style=LCS)
  chart = pygal.Bar(style=my_style, x_label_rotation=45, show_legend=False)
  chart.title = 'Most-Stared Python Project On Github'
  chart.x_labels = names
  chart.add('', plot_dicts)
  
  chart.render_to_file('new_repos.svg')
  </code></pre>
  
  <p>*If I run this, there will be an error.</p>
  
  <pre><code>Traceback (most recent call last):
    File "new_repos.py", line 54, in &lt;module&gt;
      chart.render_to_file('new_repos.svg')
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\graph\public.py", line 114, in render_to_file
      f.write(self.render(is_unicode=True, **kwargs))
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\graph\public.py", line 52, in render
      self.setup(**kwargs)
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\graph\base.py", line 217, in setup
      self._draw()
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\graph\graph.py", line 933, in _draw
      self._plot()
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\graph\bar.py", line 146, in _plot
      self.bar(serie)
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\graph\bar.py", line 116, in bar 
      metadata)
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\util.py", line 234, in decorate 
      metadata['label'])
    File "C:\Users\Cao Kangkang\AppData\Roaming\Python\Python36\site-packages\pygal\_compat.py", line 62, in to_unicode
      return string.decode('utf-8')
  AttributeError: 'NoneType' object has no attribute 'decode'
  </code></pre>
  
  <p>*How ever if I change part of the code to this, the error will disappear, *however the description will be ignored in the chart.</p>
  
  <pre><code>names, plot_dicts = [], []
  for repo_dict in repo_dicts:
      names.append(repo_dict['name'])
      plot_dict = {
          'value': repo_dict['stargazers_count'],
          #'label': repo_dict['description'],
          'xlink': repo_dict['owner']['html_url'],
          }
      plot_dicts.append(plot_dict)
  </code></pre>
  
  <p>*I don't know why, Can anyone help me with this problem?</p>