title: null column NOT IN list of strings strange results
body: <p>I am getting weird results when using a spark SQL statements like:</p>
  <pre><code>select * from mytab where somecol NOT IN ('ABC','DEF')
  </code></pre>
  <p>If I set somecol to ABC it returns nothing. If I set it to XXX it returns a row.</p>
  <p>However, if I leave the column blank, like ,, in the CSV data (so the value is read as null), it still does not return anything, even though null is not in the list of values.</p>
  <p>This remains the case even if re-written as NOT(somecol IN ('ABC','DEF')).</p>
  <p>I feel like this is to do with comparisons between null and strings, but I am not sure what to do about null column values that end up in IN or NOT IN clauses.</p>
  <p>Do I need to convert them to empty strings first?</p>