fix: Dataform: Fixes "next" button staying greyed out despite form being completed

This fixes a usage-impairing bug that I am responsible for introducing earlier (oops) when the list-single field in question is marked as "required" (not necessarily if it is the second step as I wrote in the bug report). We access a the _validate() private attribute which is not great, I think it is OK in this case, let me know what you think.

Fixes: #11920 (closed)