Fix regression with empty array

Regression noticed in this line:
3f97d98bff/ui/tabs/downloads.slint (L77)

The following used to work:

```slint
import { Button, VerticalBox } from "std-widgets.slint";
export component Demo {
    in property <[int]> mods;
    VerticalBox {
        alignment: start;
        for xxx in true ? mods : [] : HorizontalLayout { alignment: center; Button { text: "OK!"; } }
    }
}
```

But we fixed array conversion and this caused a regression with empty
array

api/cpp/include/slint.h

@@ -517,6 +517,15 @@ public:
     }
 };
 
+// Specialize for the empty array. We can't have a Model<void>, but `int` will work for our purpose
+template<>
+class ArrayModel<0, void> : public Model<int>
+{
+public:
+    size_t row_count() const override { return 0; }
+    std::optional<int> row_data(size_t) const override { return {}; }
+};
+
 /// Model to be used when we just want to repeat without data.
 struct UIntModel : Model<int>
 {