code: @NonNull
      public Expression isNullOrMissing() {
          return new UnaryExpression(this, UnaryExpression.OpType.Null)
              .or(new UnaryExpression(this, UnaryExpression.OpType.Missing));
      }
docs: Creates an IS NULL OR MISSING expression that evaluates whether or not the current
  expression is null or missing.
  
  @return An IS NULL expression.