Add empty content messages

[mikejolley]

Forces the product grid block renderer to return nothing if no products are found so that a message is displayed.

Fixes #686

Screenshots

After/if WordPress/gutenberg#16512 is merged, we get this instead:

Blocked by that PR.

How to test the changes in this Pull Request:

1. Select a category with no products and save
2. See result

Changelog

Show a message in the editor if no products are found rather than show nothing.

[nerrad]

@mikejolley I reviewed the blocking pull and added some comments that I think will help move it along. If you want to take a look and I can prioritize reviewing it. I think with a few changes there we can get it merged so that it lands at least in WP 5.3 (otherwise that delays when it will be available for Woo).

[nerrad]

Linked GB pull has been merged. Still will be some time before we can utilize it (likely won't land in wp core until WordPress 5.3)

[nerrad]

However, I guess since the enhancement for this pull involves using props, we can still add those props and they would just be ignored on earlier versions of ServerSideRender?