useApolloClientDevTools should not throw when given `undefined` as client

our usecase requires loading `client` in a useEffect after some data has loaded. e.g.
```js
useEffect(() => {
(async () => {
const _persistor = cachePersistorFactory(cache);
await _persistor.restore();
const _client = clientFactory(cache, loginProviderRef);
setCachePersistor(_persistor);
setClient(_client);
})();
}, [loginProviderRef]);
```

This means we can't use this hook because it throws an error when client is undefined (on the first render) and it's not possible to render hooks conditionally.

*Expected Behaviour:*
useApolloClientDevTools does nothing if client is undefined.