Element: Add isEmptyElement utility function

[gziolo]

Description

This PR exposed new isEmptyElement utility helper function in @wordpress/element packaged based on the suggestion from @aduth #9371 (comment):

In that case I'd rather just flag it as experimental.

My main issue is that, as far as anything in components is concerned, there should be zero awareness of what value an element holds. There is no commitment guaranteed by @wordpress/element that this shape is to remain, and will be hard to account for if ever changed. And we'd do well to avoid future headaches of plugin authors who follow similar practices to see how to accommodate this need by "solving" it internally for our own usage (as otherwise, they'll follow the same pattern and similarly face breakage whenever element structure changes).

I exposed it as stable API because it is used by @wordpress/components so I'm not quite sure if it really should be experimental.

How has this been tested?

npm test

[tofumatt]

Makes sense to me; if we're using it for Slot it's not really experimental 😄