wp-admin/includes/bookmark.php: Fix potential PHP fatal errors.

[xknown]

Updates wp_update_link to handle invalid link ids, it also adds a couple
of unit tests.

Trac ticket: https://core.trac.wordpress.org/ticket/51423

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[hellofromtonya]

Status: Blocked

The solution for this PR is currently blocked until we figure out the best way to handle wp_get_object_terms. The discussion is happening in #736. Once we land on an implementation, I can update this PR.

[hellofromtonya]

Code review discussion with @jrfnl:

• Use the same strategy as in PR 736 by flipping this around to check that wp_get_object_terms does return an array and then pass that into array_unique(); else, return an empty array().
• We do not need to return the WP_Error. Why? This function expects to return only an array.

if ( is_array( $cats ) {
	return array_unique( $cats );
}

return array();

[hellofromtonya]

Code discussion with @jrfnl:

The code assumes it $links will be an array. However, get_bookmark() could also return null which is problematic:

$link_cats = $link['link_category'];

and

$linkdata = array_merge( $link, $linkdata );

https://3v4l.org/KPmBt
https://3v4l.org/4vURI

We need to wrangle this. It does not need WP_Error.

[hellofromtonya]

Defer to the reorganize branch here and specifically here in the wp-admin/includes/bookmark.php file https://github.com/jrfnl/wordpress-develop-official/blob/reaarrange/xknown/fix/51423-wp-admin-includes-bookmark.php/src/wp-admin/includes/bookmark.php