A number of issues were discovered in 2.6.0, in particular related to the additional
validation performed on the backend.

* [#4065] Fixed validation being run for fields/components that are (conditionally)
  hidden. The behaviour is now consistent with the frontend.
* [#4068] Fixed more backend validation issues:

    * Allow empty string as empty value for date field.
    * Don't reject textfield (and derivatives) with multiple=True when
      items inside are null (treat them as empty value/string).
    * Allow empty lists for edit grid/repeating group when field is
      not required.
    * Skip validation for layout components, they never get data.
    * Ensure that empty string values for optional text fields are
      allowed (also covers derived fields).
    * Fixed validation error being returned that doesn't point to
      a particular component.
    * Fixed validation being run for form steps that are (conditionally) marked as
      "not applicable".

* [#4069] Fixed a crash in the form designer when navigating to the variables tab if you
  use any of the following registration backends: email, MS Graph (OneDrive/Sharepoint)
  or StUF-ZDS.