#### Describe the bug
On the Interceptor section on README.md, where we are explaining the usage of the newly introduced `synchronous` and `runWhen` flags on `AxiosInterceptorOptions`, we are passing null for the optional parameters for example:

```javascript
axios.interceptors.request.use(function (config) {
  config.headers.test = 'I am only a header!';
  return config;
}, null, { synchronous: true });
```

When using this in a typescript project you'll get an error saying:

> Argument of type 'null' is not assignable to parameter of type '((error: any) => any) | undefined'

The reason is we didn't specify null as a type for our interceptor handlers.

#### To Reproduce
In a typescript project with `strictNullChecks` set to `true` , import axios and then write the following in you IDE(make sure typescript is configured in your IDE) 

```javascript
axios.interceptors.request.use(function (config) {
  config.headers.test = 'I am only a header!';
  return config;
}, null, { synchronous: true });
```

Your IDE will now report the following error

> Argument of type 'null' is not assignable to parameter of type '((error: any) => any) | undefined'

As per the `typescript` docs

>When `strictNullChecks` is `true`, `null` and `undefined` have their own distinct types and you’ll get a type error if you try to use them where a concrete value is expected.

#### Expected behavior
Type definition should explicitly include `null` as a type for the interceptor methods

#### Environment
 - Axios Version [e.g. 1.1.2]
 - Adapter [e.g. XHR/HTTP]
index.d.ts
only ERR_BAD_REQUEST = "ERR_BAD_REQUEST";
  static readonly ERR_NOT_SUPPORT = "ERR_NOT_SUPPORT";
  static readonly ERR_INVALID_URL = "ERR_INVALID_URL";
  static readonly ERR_CANCELED = "ERR_CANCELED";
  static readonly ECONNABORTED = "ECONNABORTED";
  static readonly ETIMEDOUT = "ETIMEDOUT";
}
export class CanceledError<T> extends AxiosError<T> {
}
export type AxiosPromise<T = any> = Promise<AxiosResponse<T>>;
export interface CancelStatic {
  new (message?: string): Cancel;
}
export interface Cancel {
  message: string | undefined;
}
export interface Canceler {
  (message?: string, config?: AxiosRequestConfig, request?: any): void;
}
export interface CancelTokenStatic {
  new (executor: (cancel: Canceler) => void): CancelToken;
  source(): CancelTokenSource;
}
export interface CancelToken {
  promise: Promise<Cancel>;
  reason?: Cancel;
  throwIfRequested(): void;
}
export interface CancelTokenSource {
  token: CancelToken;
  cancel: Canceler;
}
export interface AxiosInterceptorOptions {
  synchronous?: boolean;
  runWhen?: (config: AxiosRequestConfig) => boolean;
}
export interface AxiosInterceptorManager<V> {
  use(onFulfilled?: (value: V) => V | Promise<V>, onRejected?: (error: any) => any, options?: AxiosInterceptorOptions): number;
  eject(id: number): void;
  clear(): void;
}
export class Axios {
  constructor(config?: AxiosRequestConfig);
  defaults: AxiosDefaults;
  interceptors: {
    request: AxiosInterceptorManager<AxiosRequestConfig>;
    response: AxiosInterceptorManager<AxiosResponse>;
  };
  getUri(config?: AxiosRequestConfig): string;
  request<T = any, R = AxiosResponse<T>, D = any>(config: AxiosRequestConfig<D>): Promise<R>;
  get<T = any, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  delete<T = any, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  head<T = any, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  options<T 