Optional attribute declared as required on typescript declaration

[GbAlien]

Version

3.1.0

Reproduction link

https://github.com/vuejs/vuex/blob/dev/types/index.d.ts

Steps to reproduce

Test a mutation without payload on typescript

What is expected?

Typescript not complain about missing payload

What is actually happening?

Typescript complains about missing payload

---

change:

export type Mutation<S> = (state: S, payload: any) => any;

to:

export type Mutation<S> = (state: S, payload?: any) => any;