FIX: ensures validValues is an array (PR #13435)

Before this fix the setting object would have exceptions on 3 fields: computedNameProperty, computedValueProperty and validValues

TypeError: Cannot read property 'forEach' of undefined
    at Class.validValues (http://localhost:4200/assets/admin.js:10468:19)
    at Class.<anonymous> (http://localhost:4200/assets/vendor.js:82492:19)
    at http://localhost:4200/assets/vendor.js:28633:34
    at untrack (http://localhost:4200/assets/vendor.js:26641:7)
    at ComputedProperty.get (http://localhost:4200/assets/vendor.js:28632:13)
    at Class.CPGETTER_FUNCTION [as validValues] (http://localhost:4200/assets/vendor.js:26259:25)
    at Class.r (<anonymous>:1:83)

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