fix(types): missing null declaration for error in subscription callback (#1589)

Author: dachrillz

types/lib/client.d.ts

@@ -68,7 +68,7 @@ export interface ISubscriptionMap {
 
 export declare type OnConnectCallback = (packet: IConnackPacket) => void
 export declare type OnDisconnectCallback = (packet: IDisconnectPacket) => void
-export declare type ClientSubscribeCallback = (err: Error, granted: ISubscriptionGrant[]) => void
+export declare type ClientSubscribeCallback = (err: Error | null, granted: ISubscriptionGrant[]) => void
 export declare type OnMessageCallback = (topic: string, payload: Buffer, packet: IPublishPacket) => void
 export declare type OnPacketCallback = (packet: Packet) => void
 export declare type OnCloseCallback = () => void