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how does @Optional work when using Kotlin? is the Property null? doesn't look like I can specify a Property&lt;String?&gt; since Property requires non nullable types

`Property&lt;String?&gt;` will fail directly

Kotlin or not is irrelevant. 
Optional means you can not configure it. 
But if you fit example have a regular file property that is optional and you do configure it, the configured file still has to exist.

Setting a property to `null` means to reset it to it's convention. Setting it to a provider that evaluates to `null` means it is unset.