Widgets Screen: Suppress the Legacy Widget block when no 3rd party widgets installed.

[garretthyder]

Is your feature request related to a problem? Please describe.
The Legacy Widget block is used to provide a dropdown allowing 3rd party widgets. When there's none available it can be added but will display a 'There are no widgets available' message.

Describe the solution you'd like
It would be nice to simply suppress the block when there's no 3rd party widgets available. If there are some and get removed we can leave the block with the existing message. But we should just suppress when it's has no application.

[talldan]

As mentioned on #29018, a user might have some Legacy Widgets (e.g. Recent Posts) that are already added to a widget area (converted from widgets to blocks), but they then can't add more of that type to a widget area.

I understand the reasons why, but this is a little confusing. Not sure if there's a better way to handle it.

[noisysocks]

Haven't investigated this but we may be able to do this simply by setting supports.inserter to false on the core/legacy-widget block when there are no widgets which can be selected.

[noisysocks]

Could potentially use blockEditor.__unstableCanInsertBlockType for this.