REST API: Ensure string returned in WP_REST_Templates_Controller::get…

…_wp_templates_author_text_field().

Adds a fail-safe to return an empty string should the `switch` ever fall through without returning.

Currently, `WP_REST_Templates_Controller::get_wp_templates_author_text_field()` is tightly coupled to `WP_REST_Templates_Controller::get_wp_templates_original_source_field()`. However, if the `$original_source` values change in either method, but not both, it is possible a `void` or `null` will be returned, rather than a `string`.

Follow-up to [57366].

Props antonvlasenko, hellofromTonya, debarghyabanerjee.
Fixes #61580.

git-svn-id: https://develop.svn.wordpress.org/trunk@58705 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php

@@ -871,6 +871,9 @@ private static function get_wp_templates_author_text_field( $template_object ) {
 				}
 				return $author->get( 'display_name' );
 		}
+
+		// Fail-safe to return a string should the original source ever fall through.
+		return '';
 	}