The getClientIp can return null (#41902)

As you can see in the source code here https://github.com/pbojinov/request-ip/blob/master/src/index.js#L129 the `getClientIp` function can return `null` value if all the possible methods fail.

types/request-ip/index.d.ts

@@ -38,7 +38,7 @@ interface Options {
     attributeName: string;
 }
 
-export declare function getClientIp(req: Request): string;
+export declare function getClientIp(req: Request): string | null;
 
 export function mw(options?: Options): (req: Request, res: any, next: any) => any;