Custom Fields Heading shows up when all meta values are serialized arrays/objects

[implenton]

_list_meta_row skips the meta rows when the value is a serialized array/objects.

When all meta values are serialized arrays/objects OR when you only have one meta with the value being serialized array/object the table heading shows up:

There is a check for protected metas before the table rendering

wp-admin/includes/meta-boxes.php:631

$metadata = has_meta($post->ID);
foreach ( $metadata as $key => $value ) {
	if ( is_protected_meta( $metadata[ $key ][ 'meta_key' ], 'post' ) || ! current_user_can( 'edit_post_meta', $post->ID, $metadata[ $key ][ 'meta_key' ] ) )
		unset( $metadata[ $key ] );
}
list_meta( $metadata );

but it does not take in account meta values, which are skipped later.

This might be considered an edge case, but it is better to be noted.

Discovered this when I reported: #37626

[implenton]

Check for serialized array/objects values early

[implenton]

Check for serialized array/objects values early - fix space between ! and &&

[implenton]

I provided a .diff that makes sure that it checks early for serialized array/object meta values and fixes the issue I reported.

[SergeyBiryukov]

Previously: #16214