Make `Query#populate()` and `Document#populate()` no-ops when populating a path with no `ref` or `refPath`, and no `model` option

Re: #8946. When using `Model.populate()`, it makes sense to fall back to the `Model` if no `ref`. But when using `Query#populate()`, especially with nested `populate()`, this behavior can lead to some nasty surprises.

Review issue #8946 and trace the Query#populate(), Document#populate(), and Model.populate() entry points, including nested populate() behavior. Confirm that Query#populate() and Document#populate() do nothing when a path has no ref or refPath and no model option, while Model.populate() retains its fallback behavior; add or update coverage for these cases.

Written by the indexing model from the issue text.