Feature: #102077 - Allow custom default value in getFormValue() conditions function

Description

The getFormValue() function can be used in conditions of form variants to safely retrieve form values. Before, null was returned as default value. This made it impossible to use this, for example, with the in operator to check values in multi-value form fields. An additional check was necessary to avoid type issues:

variants:
  - identifier: variant-1
    condition: 'getFormValue("multiCheckbox") && "foo" in getFormValue("multiCheckbox")'

A second argument has been added to this function to set a custom default value. This allows shortening conditions accordingly:

variants:
  - identifier: variant-1
    condition: '"foo" in getFormValue("multiCheckbox", [])'

Impact

Form variant conditions can be shortened.