fix(node-fetch): make systemError optional

types/node-fetch/index.d.ts

@@ -4,6 +4,7 @@
 //                 Niklas Lindgren <https://github.com/nikcorg>
 //                 Vinay Bedre <https://github.com/vinaybedre>
 //                 Antonio Román <https://github.com/kyranet>
+//                 Andrew Leedham <https://github.com/AndrewLeedham>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 /// <reference types="node" />
@@ -134,7 +135,7 @@ export class Body {
 
 export class FetchError extends Error {
     name: "FetchError";
-    constructor(message: string, type: string, systemError: string);
+    constructor(message: string, type: string, systemError?: string);
     type: string;
     code?: string;
     errno?: string;