Mark DB2Dialect as not supporting null columns in unique constraint (#2548)

Author: hazzik

src/NHibernate/Dialect/DB2Dialect.cs

@@ -297,6 +297,8 @@ public override string ForUpdateString
 
 		#region Overridden informational metadata
 
+		public override bool SupportsNullInUnique => false;
+
 		public override bool SupportsEmptyInList => false;
 
 		public override bool SupportsResultSetPositionQueryMethodsOnForwardOnlyCursor => false;