title: Replacing null in a list column pyspark
body: <p>I have a dataframe like below and trying to replace the null using df.fillna(0) or df.fillna("nn"), both seem to have no impact on the dataframe. Any advice would be greatly appreciated.
  <code>
        id    |     value
  +-----------+------------+
            1 |   ["a","b","c"]
            6 |      null</code></p>