Feature: #102077 - Allow custom default value in getFormValue() conditions function
See forge#102077
Description
The 
        get 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")'        
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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", [])'        
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Impact
Form variant conditions can be shortened.