When an empty legacy widget is added the widget area can't be saved

[draganescu]

Steps to reproduce:

• have the current master of Gutenberg
• have a default WordPress install with no extra plugins
• go to Appearance -> Widgets
• at the end of a widget area add a legacy widget block
  • the legacy widget block should say there are no available widgets
• save the page
• see the error

Screenshots

widgets-legacy-no-save.mp4

[tellthemachines]

This looks like the same error as #31249

[noisysocks]

We can fix this the same way we did in the Customzier by converting the Legacy Widget block to a block widget when it is empty.

gutenberg/packages/customize-widgets/src/components/sidebar-block-editor/use-sidebar-block-editor.js

Lines 17 to 21 in cc2e078

	const isValidLegacyWidgetBlock =
	block.name === 'core/legacy-widget' &&
	( block.attributes.id || block.attributes.instance );
	if ( isValidLegacyWidgetBlock ) {

While we're at it, we should move blockToWidget and widgetToBlock to @wordpress/widgets so that we're not defining the same functions in both edit-widgets and customize-widgets.

[noisysocks]

While we're at it, we should move blockToWidget and widgetToBlock to @wordpress/widgets so that we're not defining the same functions in both edit-widgets and customize-widgets.

Hm, can't really do this as they're different implementations. In @wordpress/customize-widgets we are mapping from blocks ↔ how widgets are represented in the Customizer. In @wordpress/edit-widgets we are mapping form blocks ↔ how widgets are represented in the REST API. They aren't the same unfortunately.