Make selectbox invisible if its size is zero

[zk-phi]

This PR makes the Selectbox invisible by setting its display property to none, iff both width and height of the box is zero.

This avoids rendering a strange dot at the top-left position of the SelectableGroup.

Thank you and regards.

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🔍 Inspect: https://vercel.com/skymk1/react-selectable-fast/4gvmvde7g ✅ Preview: https://react-selectab-git-fork-zk-phi-avoid-rendering-zerowidth-31cb18.skymk1.now.sh

[JakeAngell]

@valerybugakov please merge, simple one line fix for annoying bug