#### 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
= any, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  post<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  put<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  patch<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  postForm<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  putForm<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  patchForm<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
}
export interface AxiosInstance extends Axios {
  <T = any, R = AxiosResponse<T>, D = any>(config: AxiosRequestConfig<D>): Promise<R>;
  <T = any, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  defaults: Omit<AxiosDefaults, 'headers'> & {
    headers: HeadersDefaults & {
      [key: string]: AxiosHeaderValue
    }
  };
}
export interface GenericFormData {
  append(name: string, value: any, options?: any): any;
}
export interface GenericHTMLFormElement {
  name: string;
  method: string;
  submit(): void;
}
export function toFormData(sourceObj: object, targetFormData?: GenericFormData, options?: FormSerializerOptions): GenericFormData;
export function formToJSON(form: GenericFormData|GenericHTMLFormElement): object;
export function isAxiosError<T = any, D = any>(payload: any): payload is AxiosError<T, D>;
export function spread<T, R>(callback: (...args: T[]) => R): (array: T[]) => R;
export function isCancel(value: any): value is Cancel;
export function all<T>(values: Array<T | Promise<T>>): Promise<T[]>;
export interface AxiosStatic extends AxiosInstance {
  create(config?: CreateAxiosDefaults): AxiosInstance;
  Cancel: CancelStatic;
  Can