Dialog is rendering a close button when no onClose is provided

[lyzadanger]

Dialog supports a legacy use case of "non-closeable" modals. As such, the onClose prop is currently optional. When not provided, no close button should be rendered.

As of the addition of TransitionComponent support, there is always an internal close handler generated and, as such, a close button is always rendered. You can see a close button appearing even when no close callback is provided in one of the examples on the Dialog's pattern-library documentation page:

We shouldn't render a close button if no onClose is provided to Dialog.