types: fix mutate return type

packages/vue-apollo-composable/src/useMutation.ts

@@ -21,7 +21,7 @@ type DocumentParameter<TResult, TVariables> = DocumentNode | Ref<DocumentNode> |
 type OptionsParameter<TResult, TVariables> = UseMutationOptions<TResult, TVariables> | Ref<UseMutationOptions<TResult, TVariables>> | ReactiveFunction<UseMutationOptions<TResult, TVariables>>
 
 export type MutateOverrideOptions = Pick<UseMutationOptions<any, OperationVariables>, 'update' | 'optimisticResponse' | 'context' | 'updateQueries' | 'refetchQueries' | 'awaitRefetchQueries' | 'errorPolicy' | 'fetchPolicy' | 'clientId'>
-export type MutateResult<TResult> = Promise<FetchResult<TResult, Record<string, any>, Record<string, any>>>
+export type MutateResult<TResult> = Promise<FetchResult<TResult, Record<string, any>, Record<string, any>> | null>
 export type MutateFunction<TResult, TVariables> = (variables?: TVariables | null, overrideOptions?: MutateOverrideOptions) => MutateResult<TResult>
 
 export interface UseMutationReturn<TResult, TVariables> {
@@ -101,6 +101,7 @@ export function useMutation<
         throw e
       }
     }
+    return null
   }
 
   onBeforeUnmount(() => {