title: bucket.blob.upload_from_string returns "ValueError: None could not be converted to unicode"
body: <p>I am trying to upload a DataFrame (<code>df</code>) to cloud storage using the following line of code:</p>
  
  <pre><code>bucket.blob(gcs_reference).upload_from_string(df.to_csv(index=False, encoding='utf-8'), content_type=''application/octet-stream')
  </code></pre>
  
  <p>However, my DataFrame has for some cells None values and therefore it triggers the error:</p>
  
  <blockquote>
    <p>ValueError: None could not be converted to unicode</p>
  </blockquote>
  
  <p>Is there a way to surpress this and just put empty values for the None? I did not had any issues with this in the past, I cannot recall what I did different.</p>