Coding Standards: Remove extra check in WP_REST_Server::get_json_last_error().

`json_last_error()` can only return an integer, and the `JSON_ERROR_NONE` constant has the value of `0`, so the `empty()` check is redundant here.

Follow-up to [34928], [46206].

Props dilipbheda, audrasjb, SergeyBiryukov.
See #63249.

git-svn-id: https://develop.svn.wordpress.org/trunk@60151 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/rest-api/class-wp-rest-server.php

@@ -1339,9 +1339,7 @@ protected function respond_to_request( $request, $route, $handler, $response ) {
 	 * @return false|string Boolean false or string error message.
 	 */
 	protected function get_json_last_error() {
-		$last_error_code = json_last_error();
-
-		if ( JSON_ERROR_NONE === $last_error_code || empty( $last_error_code ) ) {
+		if ( JSON_ERROR_NONE === json_last_error() ) {
 			return false;
 		}