In [44185], a bug was introduced where hierarchical post types would not display in the correct default order (hierarchically).
This was caused by a `! isset()` check, which returned `false` after [44185], causing the correct default value to not be applied. This switches that conditional to use an `empty()` check, ignoring the new empty string assignment that was added to prevent a PHP notice when `compact()` is called.
Props davidbinda.
Fixes#45711.
git-svn-id: https://develop.svn.wordpress.org/trunk@44338 602fd350-edb4-49c9-b593-d223f7449a82