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

props nabil_kadimi.
see #28361.

git-svn-id: https://develop.svn.wordpress.org/trunk@28584 602fd350-edb4-49c9-b593-d223f7449a82
Avoid a PHP notice in bundled themes if no images were found.

props nabil_kadimi.
fixes #28361.

git-svn-id: https://develop.svn.wordpress.org/trunk@28585 602fd350-edb4-49c9-b593-d223f7449a82