fix(MutationOptions): improve the type of onMutate (#2494)

set the return type of onMutate as Promise<TContext | undefined> instead of Promise<TContext> | Promise<undefined> to make it work with async functions

Author: ICodeMyOwnLife

src/core/types.ts

@@ -481,7 +481,7 @@ export interface MutationOptions<
   variables?: TVariables
   onMutate?: (
     variables: TVariables
-  ) => Promise<TContext> | Promise<undefined> | TContext | undefined
+  ) => Promise<TContext | undefined> | TContext | undefined
   onSuccess?: (
     data: TData,
     variables: TVariables,

src/react/types.ts

@@ -77,7 +77,7 @@ export interface UseMutationOptions<
   mutationKey?: MutationKey
   onMutate?: (
     variables: TVariables
-  ) => Promise<TContext> | Promise<undefined> | TContext | undefined
+  ) => Promise<TContext | undefined> | TContext | undefined
   onSuccess?: (
     data: TData,
     variables: TVariables,