title: Null values best practices in Parquet files
body: <p>I'm trying to figure out what is the best practice if I have a string column with possible null values.<br>
  In SQL databases null is a legit value, but from reading around I've found lots of issues and people's questions about null value in parquet files.<br>
  If I want to process these parquet files later with abroad set of tools such as Drill, Spark, etc.
  what is the best approach for storing null values, as nulls or empty strings?  </p>