REST API: Preserve unknown, respect null in server-side block rendering.

- Skips validation where there is no attribute definition, but keeps the attribute value. Previously, the attribute would be omitted from the attributes passed to `render_callback`. Notably, this resolves an issue where `render_callback` cannot receive a block's `align` and `customClassName` attribute values, since these are defined as a client-side filter.
- Validates `null` as a proper value in its own right. Previously, a client implementation of a block could track `{“attribute":null}` as an explicitly empty value, and the server would wrongly initiate defaulting behavior. The new behavior will now only populate a default value if the attribute is not defined at all, including when unset in its being invalid per the attribute schema. 

Props aduth, noisysocks, youknowriad, danielbachhuber.

Merges [43918] to trunk.

See #45145 for the patch, #45098 for the original ticket.

git-svn-id: https://develop.svn.wordpress.org/trunk@44269 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/rest-api/rest-block-renderer-controller.php

@@ -319,7 +319,9 @@ class REST_Block_Renderer_Controller_Test extends WP_Test_REST_Controller_Testca
 		$block_type = WP_Block_Type_Registry::get_instance()->get_registered( self::$block_name );
 		$defaults   = array();
 		foreach ( $block_type->attributes as $key => $attribute ) {
-			$defaults[ $key ] = isset( $attribute['default'] ) ? $attribute['default'] : null;
+			if ( isset( $attribute['default'] ) ) {
+				$defaults[ $key ] = $attribute['default'];
+			}
 		}
 
 		$request = new WP_REST_Request( 'GET', self::$rest_api_route . self::$block_name );