Only mask license if needed

Fix PHP error from occurring during license check if no license present (affects extensions).

gravity-pdf-updater.php

@@ -52,7 +52,7 @@ function( $response, $context, $class, $parsed_args, $url ) {
 		$logger = \GPDFAPI::get_log_class();
 
 		$request_body = $parsed_args['body'] ?? [];
-		if ( isset( $request_body['license'] ) ) {
+		if ( isset( $request_body['license'] ) && is_string( $request_body['license'] ) && strlen( $request_body['license'] ) > 2 ) {
 			/* mask the license key, if exists */
 			$license                 = $request_body['license'];
 			$request_body['license'] = substr( $license, 0, 1 ) . str_repeat( '*', strlen( $license ) - 2 ) . substr( $license, -1, 1 );