`get_term_children()` returns an array of IDs, not objects. The docs have been updated to reflect this.

Fixes #27312.


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In `get_terms()`, leverage `get_term_children()` over `_get_term_children()` when making sure to show empty terms that have children in a hierarchical taxonomy while avoiding duplicates.

Adds unit test for `child_of` param. Adjusts unit tests for `get_terms()`.

See [27108] and [27125].
Props SergeyBiryukov.
Fixes #27123.


Built from https://develop.svn.wordpress.org/trunk@27458


git-svn-id: http://core.svn.wordpress.org/trunk@27304 1a063a9b-81f0-0310-95a4-ce76da25c4cd