Fix item selection of a nuxeo-data-table in a search form

Steps to reproduce:

1. Deploy the attached project
2. Navigate to the search AMP (Bank icon)
3. Click on the ADD button for the field "Dublincore Source"
4. Depending on the fact that WEBUI-22 will be fixed or not, use the TAB key to navigate to the popup
5. Enter a value
6. Use the TAB key to choose the OK button
7. Submit the search form
8. Observe that an error is displayed

java.lang.ClassCastException: com.fasterxml.jackson.databind.node.IntNode cannot be cast to com.fasterxml.jackson.databind.node.ArrayNode 

This is due to the fact that the form has submitted an empty value for the field AMP - actually we can see in the request that the value is ()

Expected behavior: the selected value is correctly submitted by the form