In `get_terms()`, don't set automatically `hierarchical` to `false` when...

In `get_terms()`, don't set automatically `hierarchical` to `false` when `parent => 0` is passed. The default value for `parent` is `''`.

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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git-svn-id: https://core.svn.wordpress.org/trunk@26975 1a063a9b-81f0-0310-95a4-ce76da25c4cd