Fix: Product category pages don't respect pagination and sort order
settings.

This is only reproducible if a navigation menu is registered to the
primary nav menu area but doesn't have any menu item.

The bug is introduced after this commit in WP core:
https://github.com/WordPress/WordPress/commit/a269ba85f77cb9c8c67fe623d2ff2501aca498ba

Issue #128 and issue #134.