fix(api): fix typings for 'Buffer'

Author: jwerle

api/buffer.js

@@ -662,7 +662,7 @@ Buffer.isEncoding = function isEncoding (encoding) {
   }
 }
 
-Buffer.concat = function concat (list, length) {
+Buffer.concat = function concat (list, length = undefined) {
   if (!Array.isArray(list)) {
     throw new TypeError('"list" argument must be an Array of Buffers')
   }

api/index.d.ts

@@ -589,7 +589,7 @@ declare module "socket:buffer" {
         export function isBuffer(b: any): boolean;
         export function compare(a: any, b: any): 0 | 1 | -1;
         export function isEncoding(encoding: any): boolean;
-        export function concat(list: any, length: any): Uint8Array;
+        export function concat(list: any, length?: any): Uint8Array;
         export { byteLength };
     }
     export const kMaxLength: 2147483647;