Avoid a PHP notice in adjacent_image_link() if no images were found.

props nabil_kadimi.
see #28361.
Built from https://develop.svn.wordpress.org/trunk@28584


git-svn-id: http://core.svn.wordpress.org/trunk@28409 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Avoid a PHP notice in bundled themes if no images were found.

props nabil_kadimi.
fixes #28361.
Built from https://develop.svn.wordpress.org/trunk@28585


git-svn-id: http://core.svn.wordpress.org/trunk@28410 1a063a9b-81f0-0310-95a4-ce76da25c4cd