NavigationTabButton: Fix icon.source detection

The icon.source property has url type, and can not be compared directly to an empty string.

See also: qqc2-desktop-style@e74b8530

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Continuing this line of though, the following bindings below don't make much sense either:

            implicitHeight: source ? control.icon.height : 0
            implicitWidth: source ? control.icon.width : 0

because a url source is always considered truthy.

I wonder if we should use Kirigami.Icon::valid property more?