title: How to use the PySpark CountVectorizer on columns that maybe null
body: <p>I have a column in my Spark DataFrame:</p>
  
  <pre><code> |-- topics_A: array (nullable = true)
   |    |-- element: string (containsNull = true)
  </code></pre>
  
  <p>I'm using CountVectorizer on it:</p>
  
  <pre><code>topic_vectorizer_A = CountVectorizer(inputCol="topics_A", outputCol="topics_vec_A")
  </code></pre>
  
  <p>I get NullPointerExceptions, because sometimes the topic_A column contains null.</p>
  
  <p>Is there a way around this? Filling it with a zero-length array would work ok (although it will blow out the data size quite a lot) - but I can't work out how to do a fillNa on an Array column in PySpark.</p>