fix(types): Make mutation and action payload optional in definition file (#1517)

* Make mutation payload optional in definition file

When testing a mutation without payload in TypeScript, it complains that about the missing payload, even though it's not mandatory.

This PR simply makes the payload optional.

Fixes #1491

* Make action payload optional in definition file

Author: davidsandoz

types/index.d.ts

@@ -97,15 +97,15 @@ export interface StoreOptions<S> {
   strict?: boolean;
 }
 
-export type ActionHandler<S, R> = (this: Store<R>, injectee: ActionContext<S, R>, payload: any) => any;
+export type ActionHandler<S, R> = (this: Store<R>, injectee: ActionContext<S, R>, payload?: any) => any;
 export interface ActionObject<S, R> {
   root?: boolean;
   handler: ActionHandler<S, R>;
 }
 
 export type Getter<S, R> = (state: S, getters: any, rootState: R, rootGetters: any) => any;
 export type Action<S, R> = ActionHandler<S, R> | ActionObject<S, R>;
-export type Mutation<S> = (state: S, payload: any) => any;
+export type Mutation<S> = (state: S, payload?: any) => any;
 export type Plugin<S> = (store: Store<S>) => any;
 
 export interface Module<S, R> {