Improve data types returned from empty hierarchical term queries.

When querying for 'count', ensure that 0 is returned. Otherwise,
ensure that it's an array.

Props xParham, birgire.
Fixes #42327.
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git-svn-id: https://core.svn.wordpress.org/trunk@42038 1a063a9b-81f0-0310-95a4-ce76da25c4cd