Framework: Respect null as a valid attribute type

lib/class-wp-block-type.php

@@ -141,18 +141,14 @@ public function prepare_attributes_for_render( $attributes ) {
 
 			$schema = $this->attributes[ $attribute_name ];
 
-			// Validate value by JSON schema.
+			// Validate value by JSON schema. An invalid value should revert to
+			// its default, if one exists. This occurs by virtue of the missing
+			// attributes loop immediately following. If there is not a default
+			// assigned, the attribute value should remain unset.
 			$is_valid = rest_validate_value_from_schema( $value, $schema );
 			if ( is_wp_error( $is_valid ) ) {
-				// Assigning `null` will trigger defaulting, if applicable.
-				$value = null;
+				unset( $attributes[ $attribute_name ] );
 			}
-
-			if ( is_null( $value ) && isset( $schema['default'] ) ) {
-				$value = $schema['default'];
-			}
-
-			$attributes[ $attribute_name ] = $value;
 		}
 
 		// Populate values of any missing attributes for which the block type

phpunit/class-block-type-test.php

@@ -140,6 +140,7 @@ function test_prepare_attributes() {
 			'wrongTypeDefaulted' => 5,
 			/* missingDefaulted */
 			'undefined'          => 'include',
+			'intendedNull'       => null,
 		);
 
 		$block_type = new WP_Block_Type(
@@ -160,6 +161,10 @@ function test_prepare_attributes() {
 						'type'    => 'string',
 						'default' => 'define',
 					),
+					'intendedNull'       => array(
+						'type'    => array( 'string', 'null' ),
+						'default' => 'wrong',
+					),
 				),
 			)
 		);
@@ -169,10 +174,11 @@ function test_prepare_attributes() {
 		$this->assertEquals(
 			array(
 				'correct'            => 'include',
-				'wrongType'          => null,
+				/* wrongType */
 				'wrongTypeDefaulted' => 'defaulted',
 				'missingDefaulted'   => 'define',
 				'undefined'          => 'include',
+				'intendedNull'       => null,
 			),
 			$prepared_attributes
 		);