HPCC-20885 Avoid internal error for dataset operation in case mapping

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

ecl/hqlcpp/hqlttcpp.cpp

@@ -757,6 +757,8 @@ IHqlExpression * HqlThorBoundaryTransformer::createTransformed(IHqlExpression *
     case no_sizeof:
     case no_offsetof:
         return getTransformedChildren(expr);
+    case no_mapto:
+        return NewHqlTransformer::createTransformed(expr);
     }
 
     //Unusually, wrap the expression in a thor node before processing annotations.