Added null behavior - empty string to match block

Not having it can cause match error.

src/main/scala/com/amazon/deequ/constraints/Constraint.scala

@@ -946,7 +946,7 @@ object Constraint {
             case Some(opts) =>
               opts.nullBehavior match {
                 case NullBehavior.Fail => false
-                case NullBehavior.Ignore => null
+                case NullBehavior.Ignore | NullBehavior.EmptyString => null
               }
             case None => null
           }