React: Don't throw an error when `usePGlite()` doesn't find a PGlite instance

At the moment when PGlite is loading asynchronously, the UI has to wait. Otherwise anything using `usePGlite()` causes an error:

```
Uncaught Error: No PGlite instance found, use PGliteProvider to provide one
```

I can't easily wrap it in a try/catch because this is a hook.

Could `usePGlite()` just return `null` so the UI can handle around it until the DB loads?

Thanks!