fix: add defaults to UseMutationOptions type

Author: boschni

docs/src/pages/overview.md

@@ -47,7 +47,7 @@ In the example below, you can see React Query in its most basic and simple form
 [Open in CodeSandbox](https://codesandbox.io/s/github/tannerlinsley/react-query/tree/master/examples/simple)
 
 ```js
-import { useQuery, QueryClient, QueryClientProvider } from 'react-query'
+import { QueryClient, QueryClientProvider, useQuery } from 'react-query'
 
 const queryClient = new QueryClient()
 

examples/custom-hooks/src/index.js

@@ -1,7 +1,7 @@
 /* eslint-disable jsx-a11y/anchor-is-valid */
 import React from "react";
 import ReactDOM from "react-dom";
-import { useQueryClient, QueryClient, QueryClientProvider } from "react-query";
+import { QueryClient, QueryClientProvider, useQueryClient } from "react-query";
 import { ReactQueryDevtools } from "react-query-devtools";
 
 import usePosts from "./hooks/usePosts";

examples/simple/src/index.js

@@ -1,7 +1,7 @@
 /* eslint-disable jsx-a11y/anchor-is-valid */
 import React from "react";
 import ReactDOM from "react-dom";
-import { useQuery, QueryClient, QueryClientProvider } from "react-query";
+import { QueryClient, QueryClientProvider, useQuery } from "react-query";
 import { ReactQueryDevtools } from "react-query-devtools";
 
 const queryClient = new QueryClient();

src/react/types.ts

@@ -43,7 +43,12 @@ export interface UseQueryResult<TData = unknown, TError = unknown>
 export interface UseInfiniteQueryResult<TData = unknown, TError = unknown>
   extends InfiniteQueryObserverResult<TData, TError> {}
 
-export interface UseMutationOptions<TData, TError, TVariables, TContext> {
+export interface UseMutationOptions<
+  TData = unknown,
+  TError = unknown,
+  TVariables = void,
+  TContext = unknown
+> {
   mutationKey?: string | unknown[]
   onMutate?: (variables: TVariables) => Promise<TContext> | TContext
   onSuccess?: (