title: Flink: how to write null values as empty through writeAsCsv in sink
body: <p>I have my a pojo which I create on runtime and there could be null values in the pojo object. When I try to write the object values in a CSV file with <code>dataset.writeAsCsv</code>, the following exception appears:</p>
  
  <pre><code>org.apache.flink.types.NullFieldException: Field 0 is null, but expected to hold a value.
  </code></pre>
  
  <p>In this case my integer is null. but same is the case with Date.</p>
  
  <p>Is there any way to write null values as empty back to CSV output file?</p>