Fix deprecated notice when using PHP 8.1+

[Chouby]

The issue

Running Duplicate Post with PHP 8.1 fires a deprecated notice:

Deprecated: trim(): Passing null to parameter #1 ($string) of type string is deprecated in /app/polylang/tmp/wordpress/wp-includes/taxonomy.php on line 2751

How to test?

			$post = self::factory()->post->create_and_get();
			duplicate_post_admin_init();
			$new_id = duplicate_post_create_duplicate( $post, 'draft' );

How to fix

Pass array() instead of null as second param of wp_set_object_terms().
The reason is that null is transformed to array( null ) and WP applies trim() to all items in this array.

[jrfnl]

I fully support this fix, but would love to see some unit tests added to safeguard the fix.

[enricobattocchi]

The changes make total sense, I'll fix the syntax to comply to CS rules

[enricobattocchi]

I fully support this fix, but would love to see some unit tests added to safeguard the fix.

Agree. We'll try to add tests for that method in the upcoming cycle (though integration tests would really be needed here)

[vraja-pro]

CR & AC ✅