In `_get_term_children()`, don't skip a top-level term without first including its children in the returned term list. Ironically, the call to `_get_term_children()` in `get_terms()` has a comment stating `"Make sure we show empty categories that have children."`, but it didn't work if you were retrieving top-level categories only.
All unit tests pass. Added a unit test based on the use case described in this ticket.
Fixes#26903.
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