Avoid the empty div wrapper for 'null' custom controls

[luispuig]

The situation

None element is rendered when a render*Controls prop in null but when it is a function that returns null the control div wrapper is rendered empty:

Use case: Removing the previous/next controls when they are not needed returning null depending on currentSlide and slideCount renders the empty div wrapper causing pointer-events problems.

The proposal

This PR renders the div wrapper and the element (returned by the render*Controls prop) only when the render*Controls prop is a function and that function is not returning null/false.

[sarmeyer]

Thank you for submitting this @luispuig!