Implement `Resource` for `()`

## What problem does this solve or what need does it fill?

While this is seemingly useless, when writing advanced generic code, it can be useful to allow users to specify a "dummy" resource, which will always exist.

## What solution would you like?

Implement `Resource` for `()`, and when fetching it, always return `()`.

## What alternative(s) have you considered?

For now, I'm going to define a `Dummy` resource, insert it via my plugin and suggest that users specify its value.

## Additional context

`QueryData` is implemented for `()`, which serves a similar purpose.