Editor: Prevent PHP warning when accessing icon by area.

This changeset prevents an attempt to access an array via a key that doesn't exist.
See https://github.com/WordPress/gutenberg/pull/52961.

Props aaronrobertshaw, glendaviesnz, mukesh27.
Fixes #58920.




git-svn-id: https://develop.svn.wordpress.org/trunk@56316 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/blocks/template-part.php

@@ -250,7 +250,7 @@ function build_template_part_block_instance_variations() {
 				'area'  => $template_part->area,
 			),
 			'scope'       => array( 'inserter' ),
-			'icon'        => $icon_by_area[ $template_part->area ],
+			'icon'        => isset( $icon_by_area[ $template_part->area ] ) ? $icon_by_area[ $template_part->area ] : null,
 			'example'     => array(
 				'attributes' => array(
 					'slug'  => $template_part->slug,