HFP-3851 Fix select field validation

[otacke]

When merged in, will fix the validate function of a select field to return true if the field is optional and nothing (-) is chosen.

Close to 10 years ago (!?) the select field’s validate function was amended to allow choosing - which is equivalent to not choosing anything. The new code works fine if the field is mandatory and nothing is chosen - the return value of validate is false. However, if the field is optional and nothing is chosen, currently the return value of validate is undefined.