Highlight the Cucumber core changes in v5 upgrade guide

Author: gaeljw

docs/upgrade_v5.md

@@ -5,7 +5,17 @@ If you are using Cucumber Scala 4.7.x and want to upgrade to 5.x, please note th
 Starting from version 5.x, Cucumber Scala will try to be as close as possible to the Cucumber Java implementation while remaining Scala oriented.
 This means that package names, parameters order, internal code... will be more consistent with the Java implementation.
 
-See also the [CHANGELOG](https://github.com/cucumber/cucumber-jvm/blob/master/CHANGELOG.md) from Cucumber Core.
+## Cucumber Core changes
+
+Please read the [CHANGELOG](https://github.com/cucumber/cucumber-jvm/blob/master/CHANGELOG.md) from Cucumber Core.
+
+### Empty values in Datatables
+
+Starting with Cucumber 5.x, empty values in Datatables are now represented as `null` instead of an empty string previously.
+
+Check out the [Empty values section in Transformers documentation](https://github.com/cucumber/cucumber-jvm-scala/blob/master/docs/transformers.md#empty-values) to learn how to have empty values in your Datatables.
+
+You can also consider [upgrading to Cucumber Scala 6.x](upgrade_v6.md) straight away as Cucumber Scala 6.x provides a way to map Datatables as collections of `Option`s so that you don't have to deal with `null` anyway. 
 
 ## Packages