REST API: Assign default for block renderer attributes

Previously we would pass `NULL` to `render`. While `render` provides a default `array()` value, since an explicit value is provided, it would not take effect. With this change, the endpoint assures a default value to be provided as a parameter to get the `get_item` callback that is compatible with the function signature of `WP_BlockType::render`.

Alternatively, it could be considered to have separate invocations of `render`, depending on whether the request parameter is set.

lib/class-wp-rest-block-renderer-controller.php

@@ -59,6 +59,7 @@ public function register_routes() {
 								'type'                 => 'object',
 								'additionalProperties' => false,
 								'properties'           => $block_type->get_attributes(),
+								'default'              => array(),
 							),
 							'post_id'    => array(
 								'description' => __( 'ID of the post context.', 'gutenberg' ),