Blocks: Update default value for some fields in WP_Block_Type

Related to the issue with default values for the blocks registered on the server. By using null for some fields we can treat them as undefined on the client.

See: WordPress/gutenberg#22849.
Props aduth.
Fixes #48529. 



git-svn-id: https://develop.svn.wordpress.org/trunk@47907 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-block-type.php

@@ -32,9 +32,9 @@ class WP_Block_Type {
 
 	/**
 	 * @since 5.5.0
-	 * @var string
+	 * @var string|null
 	 */
-	public $category = '';
+	public $category = null;
 
 	/**
 	 * @since 5.5.0
@@ -44,9 +44,9 @@ class WP_Block_Type {
 
 	/**
 	 * @since 5.5.0
-	 * @var string
+	 * @var string|null
 	 */
-	public $icon = '';
+	public $icon = null;
 
 	/**
 	 * @since 5.5.0
@@ -104,33 +104,33 @@ class WP_Block_Type {
 	 * Block type editor script handle.
 	 *
 	 * @since 5.0.0
-	 * @var string
+	 * @var string|null
 	 */
-	public $editor_script = '';
+	public $editor_script = null;
 
 	/**
 	 * Block type front end script handle.
 	 *
 	 * @since 5.0.0
-	 * @var string
+	 * @var string|null
 	 */
-	public $script = '';
+	public $script = null;
 
 	/**
 	 * Block type editor style handle.
 	 *
 	 * @since 5.0.0
-	 * @var string
+	 * @var string|null
 	 */
-	public $editor_style = '';
+	public $editor_style = null;
 
 	/**
 	 * Block type front end style handle.
 	 *
 	 * @since 5.0.0
 	 * @var string
 	 */
-	public $style = '';
+	public $style = null;
 
 	/**
 	 * Constructor.