Global Styles REST API endpoint: check custom CSS is included before attempting to validate (#46561)

* Global Styles REST API endpoint: check custom CSS is included before attempting to validate

* Fix linting issue

Author: andrewserong

lib/compat/wordpress-6.2/class-gutenberg-rest-global-styles-controller-6-2.php

@@ -151,10 +151,12 @@ protected function prepare_item_for_database( $request ) {
 		if ( isset( $request['styles'] ) || isset( $request['settings'] ) ) {
 			$config = array();
 			if ( isset( $request['styles'] ) ) {
-				$config['styles']    = $request['styles'];
-				$validate_custom_css = $this->validate_custom_css( $request['styles']['css'] );
-				if ( is_wp_error( $validate_custom_css ) ) {
-					return $validate_custom_css;
+				$config['styles'] = $request['styles'];
+				if ( isset( $request['styles']['css'] ) ) {
+					$validate_custom_css = $this->validate_custom_css( $request['styles']['css'] );
+					if ( is_wp_error( $validate_custom_css ) ) {
+						return $validate_custom_css;
+					}
 				}
 			} elseif ( isset( $existing_config['styles'] ) ) {
 				$config['styles'] = $existing_config['styles'];