Second attempt at fixing #29484

[carlomanf]

What?

Fixes #29484

Why?

The previous patch, #37672, does not work 100% of the time. See details.

How?

The wp_link_pages uses the $multipage global variable to determine whether to output pagination, so checking the same variable in this context should ensure that the pagination is output whenever there is anything to output and omitted when it would be empty.

I was tempted to check isset( $GLOBALS['multipage'] ), but seeing that wp_link_pages does not include such a check, I left it out.

Testing Instructions

1. Create a post using the classic editor and add the <!--nextpage--> tag
2. Verify that the pagination is output for this post
3. Through a plugin, use the content_pagination filter to add pagination to a post that does not contain the <!--nextpage--> tag
4. Verify that the pagination is output for this post
5. Create a post using the block editor and add the core/nextpage block
6. Verify that the pagination is output for this post
7. Create a post that does not have pagination
8. Verify that the pagination is omitted for this post

Screenshots or screencast

[github-actions]

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