We use collection here without checking if it's nullptr. But in CollectionCatalog::_createNewPITCollection we call initFromExisting with nullptr. This causes a segmentation fault.

At the very least we should avoid dereferencing collection if it's nullptr.

But I think we probably actually shouldn't be calling a function initFromExisting with something that's not existing. I was actually confused the first time I came across the call site because of that. So we could also instead have an `CollectionImpl::init` function that takes a read timestamp, and use that.