Add an `iter_empty` for `RelationshipTarget`.

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

Due to bevy's conventions, `Children` and similar components may not exist, and code should generally not treat them as required to exist. Therefore it is nice to be able to return an empty iterator when `Children` is missing.

## What solution would you like?

Add a `RelationshipTarget::iter_empty() -> Self::Collection::SourceIter` method.
This is supported by all `std` collections as all of them have a `Default` implementation for their iterators.

## What alternative(s) have you considered?

Don't add this method, since this might not be an universal trait for all iterators.