🐛 [open-formulieren/open-forms#5435] skip validation for hidden selectboxes

[annashamray]

fixes open-formulieren/open-forms#5435

Explanation:

When selectboxes components is placed inside a hidden fieldset then it has properties conditionallyVisible = true but
visible = false.
In the SelectBoxes.checkComponentValidity method isValid method is called, which calls Validator.validate, which uses conditionallyVisible property. Therefore we have invisible but invalid component.
The validation should be skipped in this case, so shouldSkipValidation is added before isValid

[annashamray]

It will be fixed in open-forms-sdk repo