`downstream_customized` is being set on blocks without upstreams

[kdmccormick]

While looking at recent OLX exports I've noticed that many blocks without upstreams (that is: blocks which are not from a library) have their downstream_customized fields set. This is confusing and creates unnecessary noise in the OLX.

I think the fix for this is simple. In UpstreamSyncMixin.editor_saved, we should return early if not self.upstream.

https://github.com/openedx/edx-platform/blob/8d95c32021e360525a7d91518a5f3a45f0ac1189/cms/lib/xblock/upstream_sync.py#L525

We should add a small unit test for this, too.

[kdmccormick]

FYI @bradenmacdonald

I should be able to fix this before the Ulmo cut.

[kdmccormick]

Draft fix: #37412

[bradenmacdonald]

Thanks @kdmccormick !