Programmatically using wp_insert_post with post_author=0 does not work when user is logged in

[ericdaams]

Passing a post_author of 0 to wp_insert_post does not work if the user is logged in.

Reproducible with:

add_action( 'init', function() {
	$args = array(
		'post_title' => 'test', 
		'post_author' => 0
	);

	$post_id = wp_insert_post( $args );

	echo get_post_field( 'post_author', $post_id ); // post_author is 1
});

post_author returns 1, despite post_author being set explicitly to 0.

[ericdaams]

Using isset instead of empty ensures that post_author of 0 is correctly handled.

[johnbillion]

Thanks for the patch, ericdaams!

This will need some unit tests.

[wonderboymusic]

In 34085:

In wp_insert_post(), when setting $post_author, use isset() instead of ! empty() to allow 0 to be passed as the value for $post_author.

Adds unit tests.

Props ericdaams, wonderboymusic.
Fixes #32585.