Allow for adding a relationship without a bundle.

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

Current if I am trying to spawn a relationship without specifying any additional components, the only way to do this is by spawning a unit

```rust
commands.spawn_empty().with_related::(());
```

Which feels clunky.

## What solution would you like?

A method that allows for spawning a relationships without a bundle

```rust
commands.spawn_empty().with_related_empty::();
```

And some way of doing so with the new macros, though I'm not sure how that would be possible

## What alternative(s) have you considered?

Doing it the old way as specified above