Fix language detection for draft custom post types

[charl0tee]

Problem

Draft custom post types (CPTs) were incorrectly returning null as their language due to improper post ID resolution

Root Cause

When $post->isPreview is true, the code is using the ID of the Post's parent for language detection, but:

• wp_get_post_parent_id() was returning 0 for draft posts (that has no parents)
• The original $post->ID contained the correct post ID needed for language detection

Solution

Updated the post ID resolution logic to fallback to the original post ID when no parent exists:

• $post_id = $post_parent_id ?: $post->ID;

This ensures that:

• For actual preview posts with parents: uses the parent ID (though I'm not sure it really is necessary, as the pll_get_post_language() seems to return the correct language for preview posts)
• For draft posts without parents: uses the original post ID