Change optimisticResponse type to be consistent and less restrictiv…

…e. (#6643)

Per #6390, the extra `| {}` makes certain semantically correct patterns that used to be okay in Typescript 3.7 illegal in 3.9. The difference between the two definitions of `optimisticResponse` in this file appears to have been a simple oversight when they were added in apollographql/react-apollo#3257.

src/react/types/types.ts

@@ -176,7 +176,7 @@ export interface MutationFunctionOptions<
   TVariables = OperationVariables
 > {
   variables?: TVariables;
-  optimisticResponse?: TData | ((vars: TVariables | {}) => TData);
+  optimisticResponse?: TData | ((vars: TVariables) => TData);
   refetchQueries?: Array<string | PureQueryOptions> | RefetchQueriesFunction;
   awaitRefetchQueries?: boolean;
   update?: MutationUpdaterFn<TData>;