Correctly return zero terms when offset is greater than term count

[danielbachhuber]

Reported in ​https://github.com/WP-API/WP-API/issues/2309

Thanks for the ticket and the patch. I've confirmed the issue.

Your fix doesn't go far enough. Your tests are using very large numbers for 'offset' and 'number', which is masking another bug: get_terms() is not properly trimming the array when (number + offset) > count( $terms ) > number. The fix appears to be to skip the count( $terms ) > $number check, and run all results that make it this far through array_slice(). See 35935.diff​. Could you double-check my logic?

This collection of bugs exists since [10416], when the fix for hierarchical limits in get_terms() was introduced.

Your patch doesn't have glaring issues I can see, and works against the (more limited) tests I wrote for WP-API

In 36691:

Improve 'offset' calculation when querying for hierarchical terms.

When querying for terms in hierarchical taxonomies, get_terms() initially
queries for all matching terms, and then trims the located results based on the
$number and $offset arguments passed to the function. See #8832. However,
a flaw in the original logic meant that results were failing to be trimmed
properly in cases where $offset exceeds the total number of matching terms;
in these cases, we should force an empty array.

Props danielbachhuber.
Fixes #35935.