title: JPA - Using null values in @DiscriminatorValue
body: <p>I was wondering if I could use the DiscriminatorValue to set apart two subtypes in the following manner:
  B extends A</p>
  
  <p>@DiscriminatorValue(<strong>null</strong>)
  A</p>
  
  <p>@DiscriminatorValue("Some-Value")
  B extends A</p>
  
  <p>The point is that I want to check if there is a <strong>null</strong> value in some DiscriminatorType.Char column.
  I tried writing "" (empty string) as the value and also null. Niether worked.</p>